You can apply geometry switching to any body part. The relevant lines are :-

alternateGeom lHand_2
{
name GetStringRes(1037,2)
objFile 1002 :Runtime:Geometries:maleHandsLo:lHandShape_2.obj
}

These should be placed just above the channels section for the respective body part. Look in the Poser 2 CR2's for a comparison. I don't think you can add extra hierarchy below that. The mermaid on my tutorial CD uses this method to add gills to her neck.

The genital function is similarly scripted. Its not set up using a PHI file