The source code bit mentioned above is true.
Although, they are modifiable to an extent. If you manage to get a hold of the files necessary to change the animation (I don't know them off the top of my head, as I've never worked with this myself), you could conceivably replace the Buttswing anim to something else.
However, you cannot add or remove animations from the game, nor change their names in the file (since the game's source code is required to do this fully). So even if you replaced the Buttswing anim with something like the old Moonwalk (as someone has done this before as a prototype experiment for fun), the icon would still read as "Buttswing," and everyone would see your character buttswinging, unless they also had the mod.
So, it's kind of possible, but it can't be completely re-done. At most you can just change the appearance of one of the existing animations.