modify mecanim clips

I'm planning to use a number of animation clips downloaded from the asset store on a custom character. My problem is that my character has a significantly bigger belly that the skinny guy in the animation clip which means that my characters arms goes through his belly when the idle animation clip is used (as an example)

Question: is it possible within unity to offset animation curves inside the FBX animation clips (in this case: the rotation of the shoulder joints) or do I need to import these into a 3d package and change the curves there and then re-import these into unity?