I would like to have the title (or rather "section") "Appendix" (German: Anhang) right at the top of the page as it is, but with the pdf, I tried to include as an appendix, following right away without having a forced pagebreak.
Is this possible?
Currently it looks like this: 
Thank you very much in advance!
\documentclass[12pt,a4paper,headings=standardclasses,numbers=noenddot]{scrartcl}
\usepackage{showframe}
\usepackage[left=2.50cm, right=2.50cm, top=2.50cm, bottom=2.00cm, footskip=1cm]{geometry}
\usepackage[onehalfspacing]{setspace}
\usepackage[main=ngerman, english]{babel}
\usepackage[babel, german=quotes]{csquotes}
\usepackage[ngerman]{isodate}
\usepackage[ngerman]{datetime}
\renewcommand\sectionformat{\mbox{\thesection.\enskip$|$\enskip}}% <- changed
\usepackage[ngerman]{datetime}
\RedeclareSectionCommand[
beforeskip=2\baselineskip,% changed to a positive value, if afterindent=false is set (needs version 3.26b)
afterindent=false,% < - added, needs version 3.26b
afterskip=.5\baselineskip,
tocentrynumberformat=\def\autodot{.} % replace \autodot by a fixed dot for section entries
]{section}
\RedeclareSectionCommand[
beforeskip=-1\baselineskip,
afterskip=.5\baselineskip]{subsection}
\RedeclareSectionCommand[
beforeskip=-1\baselineskip,
afterskip=.5\baselineskip]{subsubsection}
\RedeclareSectionCommand[
beforeskip=-1\baselineskip,
afterskip=.5\baselineskip]{paragraph}
\RedeclareSectionCommand[
beforeskip=-1\baselineskip,
afterskip=.5\baselineskip]{subparagraph}
\usepackage{authblk}
%----------------------------------------------------------------------------
% APPENDICES
%----------------------------------------------------------------------------
\newcommand*\appendixmore{% see the KOMA-Script documentation
\clearpage
\addsec{\appendixname}%
\renewcommand{\thesubsection}{\Alph{subsection}}%
}
%----------------------------------------------------------------------------
% FIGURES
%----------------------------------------------------------------------------
\usepackage{graphicx}
\graphicspath{ {./figures/} }
\usepackage{lscape}
%\usepackage{booktabs}
\usepackage[final]{pdfpages}
\usepackage{chngcntr}
\renewcommand{\listfigurename}{Abbildungsverzeichnis}
\renewcommand{\figurename}{Abb.}
\counterwithout{figure}{section}
\renewcommand{\listtablename}{Tabellenverzeichnis}
\renewcommand{\tablename}{Tab.}
\counterwithout{table}{section}
\begin{document}
\appendix
%\input{figures}
%\pagebreak
%\input{tables}
%\pagebreak
\includepdf[pages=1,scale=0.75,pagecommand=\thispagestyle{plain}\subsection{Auszug aus dem Tosa Nikki} \footnotesize{\textsc{Ki} (1957): \textit{Tosa Nikki} [Das Tagebuch von Tosa].}]{Iwanami1957TosaNikki.pdf}
\includepdf[pages=2,scale=0.75,pagecommand=\thispagestyle{plain}]{Iwanami1957TosaNikki.pdf}
\end{document}

You can use the pagecommand key:
\includepdf[scale=0.8,pages=1,pagecommand=\subsection{blub}]{testpdf}
– user168390 Jun 30 '19 at 18:08