Une solution, très peu élégante.
```
\documentclass{yathesis}
\usepackage{chngcntr}
\begin{document}
\makeatletter
\renewcommandtwoopt{\YAD@starred@chapter}[3][][]{%
\booltrue{YAD@nonnumbered@chapter}%
\boolfalse{YAD@nonnumbered@section}%
\boolfalse{YAD@nonnumbered@subsection}%
\boolfalse{YAD@nonnumbered@subsubsection}%
\boolfalse{YAD@nonnumbered@paragraph}%
\boolfalse{YAD@nonnumbered@subparagraph}%
\YAD@setsecnumdepth{\YAD@secnumdepth}%
\YAD@tmp@chapter[#1][#2]{#3}%
}%
\makeatother
\chapter*{Test}
\counterwithout{section}{chapter}
\section{Test}
\mainmatter
\counterwithin{section}{chapter}
% on redéfinit la commande comme dans la classe
\makeatletter
\renewcommandtwoopt{\YAD@starred@chapter}[3][][]{%
\booltrue{YAD@nonnumbered@chapter}%
\booltrue{YAD@nonnumbered@section}%
\booltrue{YAD@nonnumbered@subsection}%
\booltrue{YAD@nonnumbered@subsubsection}%
\booltrue{YAD@nonnumbered@paragraph}%
\booltrue{YAD@nonnumbered@subparagraph}%
\YAD@setsecnumdepth{none}%
\YAD@tmp@chapter[#1][#2]{#3}%
}
\makeatother
\chapter{Test2}
\section{Quoi?}
\chapter{Test3}
\section{Et alors?}
\end{document}
```
