>>2820985You can separate a mesh and then reweigh the pieces accordingly, I guess? Select faces, E->F->K/V (note that it cuts along every selected edge, duplicating every vertex you've selected; you can then join near vertices to merge them back, but be careful when doing that around UV edge areas). The knife tool can cut new edges and create new vertices ROUGHLY where you want to make a cut, but it's really crappy, better do it in Blender.
>>2820989If you mean a model that was saved in a non-standard asymmetrical pose, you can do it via TransformView by manually positioning the model properly and updating the model with the current deformed state (you can import a sample single-mesh model as reference); anyway, I doubt you can make it symmetrical without mirroring the model afterwards. Turn off ALL IK chains first thing and don't enable physics.
