\documentclass{report}

\makeatletter
\newcommand{\supervisor}[1]{%
  \gdef\@supervisor{#1}%
  \g@addto@macro\@author{%
  \ifcsname @supervisor\endcsname
    \\ \@supervisor
  \fi}
}
\makeatother

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

\begin{document}

\maketitle

\end{document}