Can I add a small stud on top of a plaster wall?
Yes, you can do that. I'd use 4" gold construction screws every 24" or so. If this results in a wavy plane, simply insert shims behind the studs near screws where needed. 

I would remove the corner bead (if present) at the jog and tape to just the plaster. This will help eliminate the "ski jump" that's often present at outside corners, especially once you add another layer of tape.