The question is a little bit unclear: I understood this way: Group some parts in the bookmark panel, but this can be a total misunderstanding of course?
Introducing a \superpart command which occurs one level above \part, therefore level=-2 is used. There are of course more options to achieve, an entry to the ToC, for example.
\documentclass{book}
\usepackage{xcolor}
\usepackage[bookmarksopen=true]{hyperref}
\usepackage{bookmark}
\newcounter{superpart}
\newcommand{\superpart}[1]{%
\cleardoublepage
\refstepcounter{superpart}%
\phantomsection%
\hypertarget{superpart::\number\value{superpart}}{}%
\bookmark[startatroot,level=-2,bold,color=red,dest=superpart::\number\value{superpart}]{#1}%
}
\begin{document}
\superpart{Superpart A}
\part{Number One}
\chapter{First chapter}
\part{Number Two}
\chapter{Second chapter}
\superpart{Superpart B}
\part{Number Three}
\part{Number Four}
\end{document}

\documentclass{...}and ending with\end{document}. – Jul 28 '15 at 14:25\part{}\chapter{}? – Sigur Jul 28 '15 at 14:33memoirclass has the command\bookfor this. – egreg Jul 28 '15 at 15:56