Possible to change joint length/position to follow blendshape without double transforms?

I need to create a setup for a human character that turns into a creature in-game that has different limb proportions (wider shoulders, longer arms etc). Is there a way to move and resize the joints (or perhaps blend the same mesh between two different skeletons) as the full body blendshape is applied without causing double transformations of the mesh? I have explored everything I can think of and am completely stumped. Thank you for any help!

It is possible with some tricks.

  • First, you change position of bones with additive pose asset.
  • Then you correct shape of the model using morph target.

This video perfectly explains the process: