In general, for personal use without distribution on a personal machine, most modellers are fine with conversions.
The best thing to do though is to contact the original author of the mesh and ask for permission to convert it across for general release in another game format.
The reason being, it's not only common courtesy, but it may bring them by to check us out. That's what happened when I asked Kier Darby for permission to convert over his low poly Whitestar to SFC... he was by for a time and made a few posts and comments. And in some cases, it may get them releasing stuff for us as well.
And in asking, the worst thing that they may say may be "No". So definitely ask if you can.
All that being said though, while you're waiting to hear back from them, email me the mesh and I'll convert it over to MOD.