This should be exactly what you asked for:
\defineprocessor[passim][right=\quad{\it passim}]
\starttext
\samplefile{ward}\index{Ward, Peter}
\samplefile{sapolsky}\index{Sapolsky, Robert}
\seeindex{passim->Ramm, Hraban}{}
\seeindex{Hraban}{Ramm, Hraban}
\page
\placeindex[check=yes]
\page
\placeindex[check=no]
\stoptext
Massi