If you check my thread(bottom of the list), you'll see that It is possible to add new animations without replacing any. Still this so far only works with dialogue and cutscenes. Combat animations are a bit trickier. While it is possible to create new combat animations, getting the game to read them is a bit of a problem. Theoretically adding new lines to combatanimations.2da will let the game read new combat animations (note: the combatanimations.2da is all numbers and I am unsure of what the numbers correspond to) . However, you'll sill need to script them.
It's just like chicken-only different.