The closest I ever got to using a creature animation on a person was 'borrowing' the Spriggan attack and heal animations from Oblivion and putting them in FO3. Amazingly it worked just fine. Although it looked really out of place.
Oh also I've replaced my "jump" animation to a somersault and the "get up off the ground" animations to a martial art-style hand spring. They're really cool and work nicely
Even just to test, backup your Meshes\Characters\_male folder (the whole lot) and copy/paste the Prime animations you want to use into there - well, the same folders. I don't remember if they all go in the _male directory or even further in - renaming any if you have to, then try it in game.
In the GECK there's a section in the NPC window where it shows all the animations (maybe an animation tab, I really don't remember). If you set up your NPC I think you should be able to even have its animations point to the LP folder instead. Or you may need to make a custom race, have custom folders, copy/paste the animations in there then in the GECK point to those.
Just guessing as I've not done a big replacement, just a few files.
:shrug: