Sandman,
I don't know if I well understood your question, but you want to copy the existing geometry from differents parts of your assembly into your skeleton model?
If yes, from your assemby, select MODIFY->MOD SKEL->select your skeleton, then FEATURE->CREATE->GEOMETRY->COPYGEOM then from COPY GEOMETRY WINDOW select SURFACE REFS->DEFINE->INCLUDE->SOLID SURF and pick the part you want to copy. You cannot select multiple parts.
But you can make an workaroud using merge procedure. In merge you can combine several parts together. Then use this part in your skeleton.
Or create a shrinkwrap.
-Hora