Is it possible to modify the skeleton.nif or skeletonbeast.nif to add a visible skeleton to it?

And have that skeleton move with the base animation skeleton, so it would pose properly?

The idea is to add a skeleton that would still be visible once an armor mesh is added to the creature, instead of being replaced. Can't modify the armor itself, because flexibility is key.

I'm 99% sure that it works without a problem. IIRC many (most?) Midas Magic creatures are all-in-one packages where the skeleton and the mesh are in one nif. I did this myself for testing animations in Nifskope (but didn't try it in game afterwards), all you have to do is copy the mesh to the scene root of the skeleton nif.

Cool, so it would already end up rigged for the animations?

Sounds like it should be easy to test, just copy the skeleton (creature) body mesh onto the animation skeleton and see what happens.

This could be useful.

yep, Ive done it.

copy paste the nitristrips into the bip01 node and it should all rig up.