\documentclass{beamer}

\usepackage{zref-xr}
\zxrsetup{toltxlabel}
\zexternaldocument*{main}

\setbeamertemplate{theorem begin}{%
  \normalfont%
  \begin{\inserttheoremblockenv}%
  {%
    \inserttheoremname%
    \ifx\inserttheoremaddition\@empty\else\ \inserttheoremaddition\fi%
  }%
}

\begin{document}
\begin{frame}
\begin{theorem}[\ref{mytheo}]
foo
\end{theorem}
\end{frame}
\end{document}