Dear giug72,thank you very much for your support request.
UMotion's main focus is to add professional animation editing to Unity. While you can use UMotion to add new bones to a character for the purpose of creating complex control rigs useful while animating, it's not possible to skin bones (= assigning vertices to them).
Rigging and skinning is something that I recommend doing in a 3D modeling application or in services like Mixamo. Mixamo is a free web service that automatically performs rigging and skinning for human like characters. As rigging/skinning is a task very independent from the rest of your game, it makes perfectly sense to do this in an external application (and is common practice).
Please let me know if you have any follow-up questions.
Customer support service by UserEcho