MIDI is a standard... It's used to communcate data between devices so they should be speaking the same language.
Only system exclusive allows MIDI to speak a different language as it allows MIDI to chat to a certain device and no others, mostly for when you need to edit patches within a synth for example.

I'm not sure what a Roland AX-7 is, however the Emax II is sampler, so in theroy you could sample any synth you like. You would then have edit the Emax to give you the 'patch' your after. There a plently of sample CD's out there.
If your after a new selection of sounds perhaps a rack mounted synth module would be in order?