You can't 'convert' the license in the sense that you simply present the SA Commercial and they'll issue you an FAA equivalent... you'll need to sit the ground exams to qualify followed by a flight test. The ground exams are a complete joke compared to the JAA license which the SA system is largely based on so very little studying will get you through those... you can combine the flight test with your training at FSI.. your check-ride on the 1900 can also be your sign-off for the commercial... or provided that you have the necessary hours an ATPL...
I suggest that you start applying at FSI as soon as possible in order to get the TSA approval.