Sorry, as mikehill said, it can't be reasonably done without re-writing the program from scratch. The reason is that .exe files that will work on a computer are in what's known as x86 machine code. However, the Prizm can only run what is called SuperH machine code, something that's entirely different (even ignoring endianess problems). Furthermore, .exe's and .g3a's are different file formats and contain different information. It's not as simple as just changing the extension. As it currently stands, the only .exe file parser currently written for the Prizm is what you might call "incomplete" if you were an optimistic person.
@Mikehill, I know of at least two different CAS systems currently being written.