To add the optional argument, you can patch \@caption which passes its contents on to \@makecaption:
\usepackage{etoolbox}% http://ctan.org/pkg/etoolbox
\makeatletter
\patchcmd{\@caption}{#3}{#2~--~#3}{}{}
\makeatother
This just inserts the second argument of \@caption (containing the optional argument to \caption as part of the main caption:

\documentclass{scrreprt}% http://ctan.org/pkg/koma-script
\usepackage{caption,graphicx}% http://ctan.org/pkg/{caption,graphicx}
\usepackage{etoolbox}% http://ctan.org/pkg/etoolbox
\makeatletter
\patchcmd{\@caption}{#3}{#2~--~#3}{}{}
\makeatother
\begin{document}
\begin{figure}
\centering\includegraphics{example-image-a}
\caption[Baz]{Foo Bar}
\end{figure}
\end{document}
Note the requirement to use a \makeatletter...\makeatother pair. For more on this see What do \makeatletter and \makeatother do?.
Alternatively, if you don't want to do this globally, you can just use
\caption[Baz]{Baz~--~Foo Bar}
which would be a local (verbatim) version of the above.
For this to work successfully with hyperref, a larger-scale replacement of contents within \@caption is required, as offered by regexpatch's \xpatchcmd* replace-all patch:

\documentclass{scrreprt}% http://ctan.org/pkg/koma-script
\usepackage{caption,graphicx}% http://ctan.org/pkg/{caption,graphicx}
\usepackage{regexpatch,hyperref}% http://ctan.org/pkg/{regexpatch,hyperref}
\makeatletter
\xpatchcmd*{\@caption}{#3}{#2~--~#3}{}{}
\makeatother
\begin{document}
\begin{figure}
\centering\includegraphics{example-image-a}
\caption[Baz]{Foo Bar}\label{myfig}
\end{figure}
See Figure~\ref{myfig} with title~\nameref{myfig}.
\end{document}