Most likely not. You're pounding through plastic. Puncturing someone with a prosthetic hand or forearm would undoubtedly be more painful. Not probable if it's a complete arm prosthesis and the user can only hit with the force of their shoulder. But if it's just a loose socket with a clawed end (like what some wheelchair users I know have), then yes, you could definitely get punched in the arm. Also worth mentioning is that if someone has their arm removed above the elbow, there's no way to attach a prosthetic limb to them. So although they could still be punched in the torso or head, this would not be effective defense against a punch to the arm. Finally, even if you did manage to connect yourself to another human being using wires and tubes, they could still get punched in the arm. The muscles and bones of the arm are very fragile and can easily be damaged by a forceful blow. In conclusion, yes, you can get punched in the arm with a prosthetic hand/forearm.