\documentclass{report}

\usepackage{etoolbox}

\makeatletter
\newcommand{\supervisor}[1]{\gdef\@supervisor{#1}}
\patchcmd{\maketitle}% <cmd>
  {\@author}% <search>
  {\@author % <replace>
   \ifcsname @supervisor\endcsname \\ \@supervisor \fi}
  {}{}% <success><failure>
\makeatother

\title{A title}
\author{An author}
\supervisor{A supervisor}

\begin{document}

\maketitle

\end{document}