(Final code Here)
This is an extension for an extended figure caption function I requested in Extended figure caption and accompanying "List of" .
The given solution worked, but I would like it to give the function the hability to handle multiple paragraphs. Hence, I duplicated the \addtocontents and \contentsline functions to respectivelly \addefcline and \efcline, differing only by the definition \long\def instead of \def, giving them the hability to handle multiple paragraphs.
As a result, it works fine, but in the current form I have to use \protect every new line I make in the \extcaption{. Is there a way to automatically protect all new paragraph ?
\documentclass[a4paper,10pt]{report}
\usepackage{fontspec}
\usepackage{lipsum}
\usepackage{graphicx}
\usepackage{caption}
% Redefinition of \addcontentsline
\long\def\addefcline#1#2#3{%
\addtocontents{#1}{\protect\efcline{#2}{#3}{\thepage}}}
% Redefinition of \contentsline
\makeatletter
\long\def\efcline#1{\csname l@#1\endcsname}
\makeatother
% Extended figure caption
\newcommand{\extcaption}[1]{% Write to .efc file
\addefcline{efc}{extfig}{{\bfseries \protect\hyperlink{fig:\thefigure}{Figure \thefigure, page~\thepage:}} #1}}
\makeatletter
\newcommand{\printextcaptions}{\@starttoc{efc}}% Read .efc file
\newcommand{\l@extfig}[2]{\noindent #1 \vspace{\parskip} \par}% How each .efc entry is handled
\makeatother
\usepackage[linktoc=all]{hyperref}
\begin{document}
\chapter{A chapter}
\section{A section}
\lipsum[1]
\begin{figure}
\includegraphics{example-image-a.pdf}
\caption{Picture of a big A}
\extcaption{The reasons why A is magnificent
\protect\begin{itemize}
\protect\item "A" like Awesome
\protect\item "A" like Amazing
\protect\end{itemize}
}
\end{figure}
\begin{figure}
\includegraphics{example-image-b.pdf}
\caption{Picture of a big B}
\extcaption{The reasons why B is awful
\protect\begin{itemize}
\protect\item "B" like Bad
\protect\item "B" like Banana
\protect\end{itemize}
}
\end{figure}
\section{Extended figure captions}
\printextcaptions
\end{document}

\unexpanded{\unexpanded{#1}}instead of#1in the definition of\extcaption. However, no hyperlink is made, because no target is defined, but I think it's a different problem. – egreg Nov 23 '15 at 00:55