i believe it would be quite difficult. if you actually look at the structure of the human brain (eg the homonculus model of the cortex) you will note that a very large portion of the brain is dedicated to just the hands/manual dexterity. Many of the current neural prosthetics seem to act by attempting to leverage the brain's use of already-existing neural pathways being used for other functions. Sort of a "turn on my microwave when i open my refrigerator"-type dependence. I think a good sign of truly independent supernumerary prosthetics would be if some users reported phantom limb sensations in their absence