How exactly would you edit a vehicle(a walker, for example)and make sure it can still use the same anims as its stock?(e.g. edit the ATTE, add stuff take away stuff, but after that it can still use ATTE anims)

I'm guessing that you have to make sure you keep the same skeleton structure and such. (I don't know how you're going to export it from .msh, so I don't know what that'll do to a skeleton)
Also, I'd advise you to keep the same proportions between the legs, since the movement may start to look awkward if you don't. (since you'll be keeping the same animations)

mswf is right only models with the same bonehirarchy can share animations

DarthD.U.C.K. wrote:mswf is right only models with the same bonehirarchy can share animations

Care to elaborate?

(That means I can't even touch it?)

you can add bones (aalya for example can use all human anims although she has additional lekubones) and change the bonescales and rotations, but the hirarchy has to stay the same (that means you have to take the old skeleton as base for your new one)
and as mswf said, you can get weird results when changing raotations and scales of bones when using the anims for original proportions