Let me get this straight.
Z4s come in 1) Roadster (Convertible) and 2) Coupe
M3s come in a 1) Convertible (not called a Roadster) and 2) Coupe
Now, there is such thing as a Z3 M Roadster and a Z3 M Coupe, but those don't fit a Z4 Roadster hood.
I think what your asking is the hood of an e46 M3 hood (Convertibels and Coupes are the same), and no, I don't think it's going to work because of the curves and size of both hoods are very much different.
Yes, everybody wants their car to be an M3, M5, or M6, but not all cars are retrofitted to magically become an M sports car.