Your best bet is to convert the soundfont to an impulse tracker instrument. (sf2/sbk to iti).
Depending on the instrument, it can be that you'll need several instruments to get the same sound.
After that, its a matter of reconstructing the midi file. You can use the import format for it, but I recommend doing it from scratch, as it will be better for editing afterwards. (import screws up where notes are placed)
EDIT: one tip though:
If you want the screwing up reduce to as much as possible, edit your midi file first, and set the tempo to exactly 125 BPM. If the tempo changes, then remove every change, so the song is a constant 125 bpm.
The reason for 125 bpm is default tempo setting in mpt.
Go to your setup screen, to the midi tab, and set: file import-> Speed = 6, pattern size = 64.
Note, the above settings are very strict, which means that the notes are being squashed to a smaller grid than midi can have. This should be great for most people, but if not, then set the patternsize back to 128 or even 256.
If done correctly, you should have a song where almost no errors occour in time shifting.
If it goes wrong, try tempo = 120 (i'm not sure what setting i used to get a good import)
When it goes wrong, notes will be out of place which creates weird sound as if the player is drunk, and requires alot of altering. (so, getting the speed well is the best thing you can aim for.)
Hope this helps.
