You can merge geosets in MdlVis (just cnp the geoset from one model to another, on an existing geoset) but if the geoset you want to add something to is attached on multiple bones, the added part will be attached randomly on some of these bones and you must re-attach it manually.
For example you have the head of the Mountain King and want to attach it on the Paladin's torso. You open the mountain king model, select the head, do Ctrl+C, open the paladin model, switch to the geoset that contains the torso, and do Ctrl + V. Then you go to Modules> Sequence Editor > Bones and attach the vertices of the head to Bone_Head.