MGIs are unfortunately different to IFFs - they're compressed. HCl has figured out a way of making uncompressed MGIs (which, I guess, would be about the same as IFFs, but I'm not sure), and then editing the .EXE to use them. The downside of this method, however, is that uncompressed MGIs are obviously quite big (therefore difficult to distribute). Furthermore, once the .EXE is edited to play uncompressed MGIs, it will apparently produce nothing but static for compressed ones.
Anyway, your best bet is to contact HCl and see what he has to say.