you can't get modplug to run in windows on a mac yet?
that'd be the way to do it I guess.
It's easier to turn a s3m file into a NSF than it is into a midi. ha ha!
the problem being that mods don't exactly work like midi files, you don't have the kind of polyphony that midis do, so a lot of trackers [like me] go a little crazy with their channels, switching what sample goes to which channel, also setting custom pitches for the samples [this is especially bad with .mod files], there are also instrument envelopes and things like that. So a direct conversion is going to sound fucking bad.
It's tough, but if you get someone to turn it into a midi for you, you're going to get back notes that most of the time will not work.
Here's an example I JUST made for this post. A midi conversion of aryx.s3m. It actually worked better than I thought but... hahaha oh wow.
http://soundtempest.net/soc/notmine/ARYX.mid
and here's the original in case you haven't heard it before:
http://uk.aminet.net/mods/s3m/aryx.lha
And if you open it in any sequencer, the timing is completely fucked.