Here is a suggestion using the KOMA-Script class scrbook but without package fncychap:
\documentclass[10pt,a4paper]{scrbook}[2017/09/07]% needs version 3.24 or newer
\usepackage[utf8]{inputenc}
\usepackage[markcase=used]{scrlayer-scrpage}
\addtokomafont{pagenumber}{\oldstylenums}
\clearpairofpagestyles%
\automark[chapter]{chapter}
\ihead{\headmark}
\ohead*{\pagemark}
\setkomafont{pagehead}{\bfseries}
\renewcommand\thechapter{\Roman{chapter}}
\KOMAoptions{chapterprefix,numbers=noenddot,egregdoesnotlikesansseriftitles}
\renewcommand\raggedchapter{\centering}
\newlength{\ChapterRuleWidth}\setlength{\ChapterRuleWidth}{2pt}
\RedeclareSectionCommand[
beforeskip=-1sp,
expandtopt,
innerskip=.5\baselineskip,
afterskip=60pt,
font=\normalfont\Huge,
prefixfont=\bfseries
]{chapter}
\newcaptionname{english}{\contributionname}{Contribution}
\DeclareNewSectionCommand[
style=chapter,
level=\chapternumdepth,
toclevel=\chaptertocdepth,
tocbeforeskip=1em plus 1pt,
tocindent=0pt,
tocnumwidth=1.5em,
tocdynnumwidth,
tocentryformat=\usekomafont{chapterentry},
tocentrynumberformat=\contributiontocnumber,
tocentrypagenumberformat=\usekomafont{chapterentry},
toclinefill=\hfill
]{contribution}
\renewcommand\thecontribution{\Roman{contribution}}
\newcommand\contributiontocnumber[1]{\contributionname\ #1:\hfill}
\renewcommand\contributionformat{\contributionname\nobreakspace\thecontribution\autodot}
\renewcommand\contributionmarkformat{\contributionname\ \thecontribution\autodot\enskip}
\automark[contribution]{contribution}
\makeatletter
\renewcommand\chapterlineswithprefixformat[3]{%
\IfArgIsEmpty{#2}{}{{%
\normalsize\rule{\linewidth}{3\ChapterRuleWidth}\*[-.5\baselineskip]%
\rule{\linewidth}{\ChapterRuleWidth}\par\nobreak
}#2}%
{\normalsize\rule{\linewidth}{\ChapterRuleWidth}\par\nobreak}
#3
\IfArgIsEmpty{#2}{\vskip -20pt}{}%
}
\makeatother
\newcounter{defaulttocdepth}
\newcommand\StartContributions{%
\cleardoublepage
\RedeclareSectionCommand[counterwithin=contribution]{section}%
\addxcontentsline{toc}{part}{Contributions}%
\addtocontents{toc}{\protect\value{tocdepth}=\contributionnumdepth}%
}
\setkomafont{partentrypagenumber}{\nullfont}
\newcommand\EndContributions{%
\RedeclareSectionCommand[counterwithin=chapter]{section}%
\addtocontents{toc}{\protect\value{tocdepth}=\the\value{tocdepth}}%
}
\usepackage{hyperref}
\usepackage{lipsum}% only for dummy text
\begin{document}
\tableofcontents
\chapter{First chapter}
\lipsum[1]
\section{First section}
\lipsum[1]
\section{Sec. section}
\lipsum[1]
\chapter{Sec. chapter}
\lipsum[1]
\section{First section}
\lipsum[1]
\section{Sec. section}
\lipsum[1-4]
\StartContributions
\contribution{First paper's title}
\lipsum[1]
\section{First section}
\lipsum[1]
\section{Sec. section}
\lipsum[1-4]
\contribution{Second contribution}
\lipsum[1]
\section{First section}
\lipsum[1]
\section{Sec. section}
\lipsum[1-4]
\EndContributions
\chapter{Test}
\section{Test}
\lipsum
\end{document}
Note that the code above needs at least KOMA-Script version 3.24. Update: If you use KOMA-Script version 3.39 or newer replace the deprecated option egregdoesnotlikesansseriftitles by the new class option sfdefaults=false.
If you want/have to use one of the versions 3.21, 3.22 or 3.23 add
beforeskip=-1sp,
innerskip=.5\baselineskip,
afterskip=60pt,
font=\normalfont\Huge,
prefixfont=\bfseries,
pagestyle=plain,
to the options of \DeclareNewSectionFormat[...]{contribution}.
Result:


But it is also possible to use package tocbasic with another class to get the desired result:
Add
\usepackage{tocbasic}
\newcommand*\Contribution{%
\renewcommand*{\chaptername}{Contribution}%
\setcounter{chapter}{0}%
\addtocontents{toc}{%
\bigskip \noindent\textbf{\large Contributions}\protect\par
\DeclareTOCStyleEntry
[entrynumberformat=\nullfont,numwidth=0pt,pagenumberformat=\textbf]
{tocline}{chapter}%
\protect\value{tocdepth}=0%
}%
\renewcommand\chaptermarkformat{}%
}
to the preamble and use \Contribution to the document body. This should also work with your special class mentioned in a comment.
Assuming that there is no switch back to normal chapters needed, I did not define an command for the end of the contributions like in my first example. But the example corrects the header entries, too.


Code:
\documentclass[10pt,a4paper]{book}
\usepackage[utf8]{inputenc}
\RequirePackage{etoolbox}
\RequirePackage{xparse}
\RequirePackage{scrhack} %
\RequirePackage[markcase=used]{scrlayer-scrpage}
\addtokomafont{pagenumber}{\oldstylenums}
%\providepairofpagestyles{thesisSimple}{%
\clearpairofpagestyles%
\automark[chapter]{chapter} %\automark[right side]{left side}
\ihead{\headmark}% Inner header
\ohead[\pagemark]{\pagemark}% Outer header
%vll
\setkomafont{pagehead}{\bfseries}
\renewcommand\thechapter{\Roman{chapter}}
\usepackage[Conny]{fncychap} %
\ChNameAsIs
\ChTitleAsIs
\makeatletter
\def@makechapterhead#1{%
\vspace*{-20\p@}
{\parindent \z@ \raggedright \normalfont
\ifnum \c@secnumdepth >\m@ne
\if@mainmatter
\DOCH
\fi
\fi
\interlinepenalty@M
\if@mainmatter %
\DOTI{#1}%
\else%
\DOTIS{#1}%
\fi
}}
\def@makeschapterhead#1{%
{\parindent \z@ \raggedright
\normalfont
\interlinepenalty@M
\DOTIS{#1}
\vskip -20\p@
}}
\makeatother
%========================
\usepackage{tocbasic}
\newcommand\Contribution{%
\renewcommand{\chaptername}{Contribution}%
\setcounter{chapter}{0}%
\addtocontents{toc}{%
\bigskip \noindent\textbf{\large Contributions}\protect\par
\DeclareTOCStyleEntry
[entrynumberformat=\nullfont,numwidth=0pt,pagenumberformat=\textbf]
{tocline}{chapter}%
\protect\value{tocdepth}=0%
}%
\renewcommand\chaptermarkformat{}%
}
\usepackage[hypertexnames=false]{hyperref}
\usepackage[english]{babel}
\usepackage[autostyle=true]{csquotes}
\usepackage{lipsum}
\begin{document}
\tableofcontents
\chapter{First chapter}
\lipsum[1]
\section{First section}
\lipsum[1]
\section{Sec. section}
\lipsum[1]
\chapter{Sec. chapter}
\lipsum[1]
\section{First section}
\lipsum[1]
\section{Sec. section}
\lipsum[1-4]
\Contribution
\chapter[Contribution I: Paper title]{First paper's title}
\lipsum[1]
\section{First section}
\lipsum[1]
\section{Sec. section}
\lipsum[1-4]
\chapter[Contribution II: Paper title]{Second contribution}
\lipsum[1]
\section{First section}
\lipsum[1]
\section{Sec. section}
\lipsum[1-4]
\end{document}
Update (appendix after contributions)
In a comment OP asks for changeing back to the normal entry format for appendix chapters.
Then I would use
\usepackage{tocbasic}
\newcommand*\Contribution{%
\renewcommand*{\chaptername}{Contribution}%
\setcounter{chapter}{0}%
\addtocontents{toc}{%
\bigskip \noindent\textbf{\large Contributions}\protect\par
\DeclareTOCStyleEntry
[entrynumberformat=\nullfont,numwidth=0pt]
{tocline}{chapter}%
\protect\value{tocdepth}=0%
}%
\renewcommand\chaptermarkformat{}%
}
\xapptocmd\appendix{%
\addtocontents{toc}{%
\defaultchaptertocstyle
\protect\value{tocdepth}=\the\value{tocdepth}%
}%
\let\chaptermarkformat\defaultchaptermarkformat
}
\newcommand*\defaultchaptertocstyle{%
\DeclareTOCStyleEntry[entrynumberformat=\bfseries,numwidth=1.5em,pagenumberformat=\textbf]
{tocline}{chapter}%
}
\AtBeginDocument{%
\let\defaultchaptermarkformat\chaptermarkformat
\defaultchaptertocstyle
}

Code:
\documentclass[10pt,a4paper]{book}
\usepackage[utf8]{inputenc}
\RequirePackage{xparse}% loads etoolbox
\RequirePackage{scrhack} %
\RequirePackage[markcase=used]{scrlayer-scrpage}
\addtokomafont{pagenumber}{\oldstylenums}
%\providepairofpagestyles{thesisSimple}{%
\clearpairofpagestyles%
\automark[chapter]{chapter} %\automark[right side]{left side}
\ihead{\headmark}% Inner header
\ohead[\pagemark]{\pagemark}% Outer header
%vll
\setkomafont{pagehead}{\bfseries}
\renewcommand\thechapter{\Roman{chapter}}
\usepackage[Conny]{fncychap} %
\ChNameAsIs
\ChTitleAsIs
\makeatletter
\def@makechapterhead#1{%
\vspace*{-20\p@}
{\parindent \z@ \raggedright \normalfont
\ifnum \c@secnumdepth >\m@ne
\if@mainmatter
\DOCH
\fi
\fi
\interlinepenalty@M
\if@mainmatter %
\DOTI{#1}%
\else%
\DOTIS{#1}%
\fi
}}
\def@makeschapterhead#1{%
{\parindent \z@ \raggedright
\normalfont
\interlinepenalty@M
\DOTIS{#1}
\vskip -20\p@
}}
\makeatother
%========================
\usepackage{tocbasic}
\newcommand\Contribution{%
\renewcommand{\chaptername}{Contribution}%
\setcounter{chapter}{0}%
\addtocontents{toc}{%
\bigskip \noindent\textbf{\large Contributions}\protect\par
\DeclareTOCStyleEntry
[entrynumberformat=\nullfont,numwidth=0pt]
{tocline}{chapter}%
\protect\value{tocdepth}=0%
}%
\renewcommand\chaptermarkformat{}%
}
\xapptocmd\appendix{%
\addtocontents{toc}{%
\defaultchaptertocstyle
\protect\value{tocdepth}=\the\value{tocdepth}%
}%
\let\chaptermarkformat\defaultchaptermarkformat
}
\newcommand*\defaultchaptertocstyle{%
\DeclareTOCStyleEntry[entrynumberformat=\bfseries,numwidth=1.5em,pagenumberformat=\textbf]
{tocline}{chapter}%
}
\AtBeginDocument{%
\let\defaultchaptermarkformat\chaptermarkformat
\defaultchaptertocstyle
}
\usepackage[hypertexnames=false]{hyperref}
\usepackage[english]{babel}
\usepackage[autostyle=true]{csquotes}
\usepackage{lipsum}
\begin{document}
\tableofcontents
\chapter{First chapter}
\lipsum[1]
\section{First section}
\lipsum[1]
\section{Sec. section}
\lipsum[1]
\chapter{Sec. chapter}
\lipsum[1]
\section{First section}
\lipsum[1]
\section{Sec. section}
\lipsum[1-4]
\Contribution
\chapter[Contribution I: Paper title]{First paper's title}
\lipsum[1]
\section{First section}
\lipsum[1]
\section{Sec. section}
\lipsum[1-4]
\chapter[Contribution II: Paper title]{Second contribution}
\lipsum[1]
\section{First section}
\lipsum[1]
\section{Sec. section}
\lipsum[1-4]
\appendix
\chapter{First appendix}
\section{An appendix section}
\lipsum
\section {Second appendix section}
\lipsum
\end{document}
tocbasicwith another class, too. – esdd Dec 06 '17 at 16:44\chapter*{Conclusion} \phantomsection\addchaptertocentry{Conclusion}\markboth{Conclusion}{Conclusion}and then\appendixand\chapter{Appendix}with its sections. Now I have problem only in the appendix whereA. Appendix titleis replaced byAppendix title– Mohamed Vall Dec 06 '17 at 20:57\addchapcommand. So you can replace your code regarding "Conclusion" by\addchap{Conclusion}. It adds the chapter entry and sets the marks for the header automatically. – esdd Dec 06 '17 at 21:30addchapno longer works proprely. – Mohamed Vall Dec 06 '17 at 22:02