How can I change the text (not the label) for each item in an itemize environment using wrappers?

I get the impression that what you're really trying to achieve by placing the arguments of \item directives in a \parbox is to re-enable hyphenation. If this impression is correct, i.e., if the main objective is hyphenation-related, there's a far simpler solution: Load the ragged2e package and issue the directive \RaggedRight immediately after \begin{itemize} (or \begin{enumerate}).

\documentclass{beamer}
\usepackage[ngerman]{babel}
\usepackage{ragged2e}
\begin{document}

\begin{frame}[t]
\begin{itemize} \RaggedRight
\item Speicherverwaltungsadressen 
      Donaudampfschifffahrtsgesellschaftsmitarbeiters
      Rechtsschutzversicherungsgesellschaftsvertrag
      sechshundertsechsundsechzigtausendzweihundertsiebenundachzig 
\end{itemize}
\end{frame}
\end{document}