You can't edit default player animations. You could make a new player type but that would be a lot of unnecessary work.
Not correct but it wouldn't be a good idea. Baddy might get all pissy at me but you can set it to read only. Read only = not replaced. I don't suggest doing this though. Skewing with the game can cause problems!
There's a thread on modification discussion where people have tried to create new player animations for months with no success.
The only problem is that they're attempting to fix something horribly broken.
Just did it... Just edited the shift one to do it for testing and it worked... They are just not trying hard enough