Sure it will. If it was easy, anyone could do it. This isn't Chapman's first rodeo. He's started in Cuba and in the minors. He has to have some idea on how to pace himself and throw another pitch.
As for a 3rd pitch, there have been scores of pitchers who were able to be successful with a great fastball and a breaking pitch or changeup. It would be nice if he could master a 3rd pitch but as hard as he throws, it may not be necessary.
I wonder what would happen if he just decided to throw a BP fastball up to the plate once or twice a game.