I am curious about this as well...while I didn't buy my DSI synth for the sequencer, I had imagined that moving MIDI files in and out of the box would be straightforward (like even, e.g., composing on Sibelius, saving as a *.mid, and then loading it into a voice).
Is there some fundamental reason why this isn't possible, or just because nobody has done it? I can imagine the possibilities...for example, using Melodyne to capture cool riffs and then re-voice them on a synth...
I bought the SoundTower SoundEditor thinking that their "Sequencer" function would help, but it turns out to be no more than a buggy sequencer attached to a simple patch librarian (although reasonably good at that).