I think most of the custom Mechs that are made or modifications of the originals (and their stats) and not entirely new geometry (the 3D models are still the same). Though I could be wrong on this. You can look at the Mechs for MechCommander 2 in the Data\TGL folder (They are .ase files) these .ase files are ASCII exports from 3D Studio MAX you could try and rebuild something in the same format but without some type of guidelines, it would be a very trial and error process. You can also look at the .ini files that go with the Mechs to see how they would be processed by ASEConv.exe which the source to is included as well.
I dunno if anyone over at this site might be able to provide some more info but it looks like they have a lot of information on MechCommander 2 http://www.mektek.net/
Hope this helps some.