\documentclass{report}

\makeatletter
\newcommand{\supervisor}[1]{\gdef\@supervisor{#1}}

\renewcommand{\maketitle}{
\begin{titlepage}
\center
  {\Huge \strut\@title} \\
  \@author
  \ifcsname @supervisor\endcsname
  \\ \@supervisor \fi \\
  \vfill
  \@date
\end{titlepage}
}
\makeatother

\author{Kajsa Eriksson}
\title{My Title}
\date{\today}
\supervisor{My supervisor}

\begin{document}

\maketitle

\end{document}