The compilers that Macromedia QA used to create and test each XDK are listed in the XDK's ReadMe. Usually, later versions of the supported compilers work, but may require some tweaking. Search the Xtras Developers area for updated information.
Developers wonder whether they can use other compilers and languages.
If your desired compiler can generate a COM DLL (aka ActiveX DLL), then in theory you can coerce it to generate an Xtra. Xtras use Microsoft's COM vtable layout, with some MOA-specific entry points. All the code needed to create an Xtra is generated by the MOA header files, particularly <moaxtra.h>. But as you can tell if you look at <moaxtra.h>, it has a ton of preprocessor macro stuff in it.
If you want to try to get the MOA headers to work for a different compiler do the following:
Faced with this research project, nearly everyone has given in and bought MSVC or CodeWarrior, because time is money.