If you want to use some Visio stencil(s) under Dia or Kivio, you need to convert it.
For help with conversion of stencils of version 6 (visio2k and visio2k2) and 11 (visio2k3) you can use vsdump.
Most of 'commercial' stencils made of a set of WMF and/or
While vsdump can't convert VSS directly to Dia or Kivio stencil, it can extract all those WMF/EMFs.
It's not quite easy to preview WMF/EMFs (e.g. EOG doesn't support this), so vsdump extracts an icons of 'Masters' and saves them in .ICO files.