I'm not sure this is a good idea. Maybe I'm wrong but from my understanding, panning in soundfonts would normally be additive, i.e. if the soundfont patch is panned somewhat to the left and the MIDI channel is panned center, then the result would be panned somewhat to the left, but if the MIDI channel is panned a bit to the right then the result would be a centered channel again. This is not how panning in modules (MPTM files in particular works): If an instrument/sample has panning enabled, it will completely override the channel panning that may have previously been set. So in a MIDI import, this would render MIDI CC 10 (pan position) useless for such instruments. This is the reason why panning is imported for reference purposes, but not enabled by default.

A more viable approach might be to import this setting as a panning envelope, since those are additive (i.e. they don't overwrite the channel panning).

Your understanding of inst/sample pan settings, pan envelopes, and the way soundfont instruments affect panning on the midi channel they're played is all correct as far as i've tested (with the nuance that in mptm's case, if the channel then switches to an unpanned instrument/sample, it will go right back to the previously set panning, something that was a bit ambiguous in the wording). I didn't really give myself the full picture of how exactly these things interacted because at the time that felt a little bit beside the point of what i was interested in. I wasn't too enthused about this exact solution either but it was all i could come up with within the bounds of what i could see was doable.

It sounds like the right solution to this would take more than a tweak then, a pretty substantial addition/rework to the way instrument envelopes work in fact, since as far as i can tell there can only be one volume/pitch/pan/filter envelope per instrument, and this setting would have to be per region. More work and change than i really would be willing to ask for in service of something that it seems few would actually use/care about, though i'm always in favor of closer accuracy for imported instruments and greater flexibility for custom instruments as long as it doesn't make the UI needlessly complicated. Maybe i'll try to muck around in the code at some point and try to come up with an agreeable solution, though i can't say it would necessarily be happening anytime soon.

EDIT: just to note for my own reference if nothing else, the panning in soundfonts goes like this: a region's pan fully overrides the instrument's if both are set, but an instrument's (and a region's; if a region's pan is unset but the global value is it just gets filled in, so to speak) is additive to the preset in the same manner it is to the channel's pan (i.e, +20 and -20 would cancel out). So it would actually likely not be too hard to import an instrument's global panning value to the pan envelope, but i don't know that that would really be of use to anyone.