How to change the label of one item in an enumitem list

I am not sure whether this is, what you want to achieve, but you can explicitly set the label of an item.

\documentclass[a4paper,10pt]{article}

\usepackage{enumitem}


\begin{document}
Foobar

\begin{enumerate}[label={(\arabic*)}]
        \item baz
        \item[\refstepcounter{enumi}(*\number\value{enumi})] xyzzy
        \item quux
\end{enumerate}

Lorem ipsum
\end{document}

I suppose, the \refstepcounter statement inside the [] is necessary because the counter is not advanced if one sets the label by hand.

Put the asterisk in the specification of the label:

\documentclass[a4paper,10pt]{article}

\usepackage{enumitem}

\newcommand{\staritem}{\global\asterisktrue\item}
\newcommand{\perhapsasterisk}{%
  \ifasterisk*\global\asteriskfalse\fi
}
\newif\ifasterisk

\begin{document}
Foobar

\begin{enumerate}[label={(\protect\perhapsasterisk\arabic*)}]
\item baz
\staritem xyzzy
\item quux
\staritem xxx
\end{enumerate}

Lorem ipsum
\end{document}