control \spaceskip

addtospaceskip.tex

\def\addtospaceskip{\@testopt{\@addtospaceskip}{}}
\def\@addtospaceskip[#1]#2{%
  \spaceskip\fontdimen2\font plus \fontdimen3\font minus \fontdimen4\font\relax
  \advance\spaceskip#2\relax
  \def\@tmp@xspaceskip{#1}%
  \xspaceskip\fontdimen7\font\relax
  \ifx\@tmp@xspaceskip\@empty
    \advance\xspaceskip#2\relax
  \else
    \advance\xspaceskip\@tmp@xspaceskip\relax
  \fi}