Well, one might be well-served to ask where tilt-rotor pilots come from now (those that aren't already pilots). U.S. Marine Corps flight students last I knew went through a somewhat hybrid pipeline with primary single-engine airplane, a somewhat abbreviated advanced helicopter syllabus, and a somewhat abbreviated multiengine airplane syllabus. I would imagine that they're able to sit for and pass Commercial exams for both single & multi engine fixed, rotary and instrument prior to heading to the tilt-rotor fleet replacement squadron.
Anecdotal information from the Marine Corps some years ago tended to suggest that for pilots beyond ab initio doing transitions fixed-wing pilots were easier to untrain.
