TeX macro \kst \kul and \khl which make kanji-characters breakable inside \st \ul and \hl of soul package. Incomplete, but works well.

ksoul.sty

\ProvidesPackage{ksoul}[2010/12/24 Kanji Based soul]
\RequirePackage{soul}[2003/11/17]

% for \st
\def\kst#1{%
  \@kst#1\end}
\def\@kst#1{%
  \ifx#1\end \let\next=\relax \else
\st{#1}%
  \let\next\@kst
  \fi\next}

% for \ul
\def\kul#1{%
  \@kul#1\end}
\def\@kul#1{%
  \ifx#1\end \let\next=\relax \else
\ul{#1}%
  \let\next\@kul
  \fi\next}

% for hl
\def\khl#1{\kul{#1}}
\def\xkhl#1{%
  \@khl#1\end}
\def\@khl#1{%
  \ifx#1\end \let\next=\relax \else
\hl{#1}%
  \let\next\@khl
  \fi\next}