\documentclass[preview,border=2mm]{standalone}

\newcommand\SpreadSpace[1]
  {%
    \begingroup
    \setbox0\hbox{$\:$ \hbox{}}%
    \spaceskip\wd0 plus \fontdimen3\font minus \fontdimen4\font
    #1%
    \endgroup
  }

\newcommand\ExpandableText{This is text}

\begin{document}
This is text\par
\SpreadSpace{This is text}\par
\SpreadSpace{\ExpandableText}
\end{document}

\documentclass[preview,border=2mm]{standalone}

\newdimen\CisTmpDimen

\newcommand\SpreadSpace[1]
  {%
    \begingroup % the group is only necessary because of the \setbox0 thingy
    \setbox0\hbox{$\:$ \hbox{}}%
    \CisTmpDimen=\fontdimen2\font % changes are made global, we have to manually revert them
    \fontdimen2\font=\wd0
    #1%
    \fontdimen2\font=\CisTmpDimen
    \endgroup
  }

\newcommand\ExpandableText{This is text}

\begin{document}
This is text\par
\SpreadSpace{This is text}\par
\SpreadSpace{\ExpandableText}
\end{document}

\documentclass[preview,border=2mm]{standalone}

\usepackage{xparse}
\ExplSyntaxOn
\tl_new:N \l_cis_tmp_tl
\NewDocumentCommand \SpreadSpace { s m }
  {
    \IfBooleanTF { #1 }
      { \tl_set:Nx \l_cis_tmp_tl { #2 } }
      { \tl_set:Nn \l_cis_tmp_tl { #2 } }
    \tl_replace_all:Nnn \l_cis_tmp_tl { ~ } { \ensuremath { \: } ~ }
    \l_cis_tmp_tl
  }
\ExplSyntaxOff

\newcommand\ExpandableText{This is text}

\begin{document}
This is text\par
\SpreadSpace{This is text}\par
\SpreadSpace{\ExpandableText}\par
\SpreadSpace*{\ExpandableText}
\end{document}