it2vgm.py is a tool for making .vgm files from .it files. You will need an .it tracker. You will also need Python.

Apparently Modplug hates the samplepack generated by smspackgen.py so if you have issues then load/resave the pack in Schism Tracker or something.

Rules for the song produced:
* First three channels can only take square waves, fourth channel can only take "noise".
* Sample indices must be EXACT and APPROPRIATE TO THE CHANNEL, and sample volumes are IGNORED.
* Lowest note is A-0, anything lower will be clipped.
* Panning is not supported (hence no Game Gear stereo).
* Voleffects are not supported.
* Format MUST be: old effects OFF, compatible Gxx OFF, AMIGA slides, MONO, SAMPLE mode. Any deviation from this WILL BE IGNORED.
* For the noise channel, any note which isn't A-4, A-5, or A-6 will mean "link this channel's period to the third square wave channel".

Links:
* it2vgm.py donload