The following example changes \setcitestyle locally and puts it in macro \citen.
\setcitestyle of package natbib (2010/09/13 8.31b) contains a space right at the beginning of the macro definition by an uncommented end of line, the trick with \romannumeral removes this space.
\documentclass{article}
\usepackage[square]{natbib}
\setcitestyle{super}
\newcommand*{\citen}[1]{%
\begingroup
\romannumeral-`\x % remove space at the beginning of \setcitestyle
\setcitestyle{numbers}%
\cite{#1}%
\endgroup
}
\begin{document}
Package \textsf{accsupp}\cite{oberdiek:accsupp}.
This figure is reproduce from reference~\citen{oberdiek:zref}.
\bibliographystyle{plainnat}
\raggedright
\bibliography{oberdiek-bundle}
\end{document}

Inside moving arguments (e.g., \caption) macro \citen can be protected using \protect or by using \DeclareRobustCommand for the definition:
\newcommand*{\citen}{}% generate error, if `\citen` is already in use
\DeclareRobustCommand*{\citen}[1]{%
\begingroup
\romannumeral-`\x % remove space at the beginning of \setcitestyle
\setcitestyle{numbers}%
\cite{#1}%
\endgroup
}