\begin{tikzpicture}
\begin{umlstate}[name=state]{I am a state}
\begin{umlstate}[name=substate1]{I am a substate 1}

\end{umlstate}
\begin{umlstate}[x=3, y=-3, name=substate2]{I am a substate 2}

\end{umlstate}
\end{umlstate}
\end{tikzpicture}