It would accept anything, provided the model format was otherwise accurate, but the issue is that if you want to use the existing supermdel and don't want to replace every single animation in the game yourself, you need the original rig.
With the MMO rig, that's not so much an issue. The bones are mostly in the same place, although the proportions are a little off. I forget if I tried retargeting the bones themselves, but I did manage to set up a rig to pose an MMO model into KOTOR size and position. But it still needed some tweaking and would have to be repeated for each body size.
As for the Selkath and other animation stuff, we last left it in the state where it looked doable, but I think it would take a lot of time and effort for someone with my level of ineptitude and I have a million other things to do.
I remember there were three big hurdles that I anticipated. First, the reach of the arms was a little - like when holding a melee weapon, the left hand didn't quite reach and grip the weapon. So that means all the combat animations would possibly have to be adjusted manually. Second, I noticed the key frame numbers were off sometimes, maybe due to rounding errors. If they got too out of sync, then the whole timeline would need to be adjusted so the animation times match the actual key frames again. Finally, there are things that can't be retargeted at all, like the eyes and any bones on the model that aren't present on the human rig (on the Selkath, for example, the flappy cephalic nodes) and those would always have to be manually animated.
I can provide a library of thousands of free sourced animations for anyone who does manage it, though. One of the other goals of messing with the MotionBuilder stuff was the potential of getting new animations from anywhere onto the KOTOR rig.