But, General MIDI is the polyphonic kind of MIDI. If you used the monotonic kind of MIDI then the channel allocation is work OK. Instrument/samples can be loaded by defining system exclusive messages to upload it with. The controls can be define as the one supported in the command column of XM format so that it is the compatible way. Timing is still the problem, unless you calculate all of the timings and figure out what speed/tempo is needed or if you specify the timing as part of the conversion or if you use the MIDI frame timing commands.
Such thing is probably not very useful in most cases, but might be useful if you want to use a external MIDI device (it could be like a organ with multiple manuals for different group of channels) to enter the music and controls and then you can add the instruments afterward with another MIDI editor, and then use a conversion program. Nevertheless, it doesn't quite make it impossible.
None of this implies that OpenMPT needs to be changed or that such files need to be put into this MOD archive; my answering is the independent question. (Nor does it necessarily do what 8bitcomputerkid wants, but, they didn't quite specify.)