Change MeshFilter with additional bones.

(Sorry, English is not my first language and i am not a programmer/animator, so some techical terms I speak about might be incorrect).

What i need: I want to be able to customize player model, for example I want to change his shirt.

What I do: I import FBX file with humanoid rig (Let's call it My_Default_Rig). Then I import separatly made clothes elements riged to My_Default_Rig , and when I want to change shirt in game i simply change MeshFilter with new shirt MeshFilter.

My problem: For exaple I want to make some more global customization. For example I want to make player more chubby (add belly). This means that I add belly bone to My_Default_Rig in Blender , slightly change my running animation so belly start bounce when player running, rig it with shirt with mesh and import it to Unity. But because my new rig is slightly different from My_Default_Rig (which is part of player GameObject), I can't just change MeshFilter. How I can solve this problem?