This is an old, old question. Please do a "search" before asking or making recommendations in teh future so as to avoid repeating ourselves.
Here is the abridged version of the answer:
a) 3DGS is DX based so a Mac port is not possible. It would have to be a complete re-write.
b) There are Win emulators which allow you to run Windows games in Macs thus making the lack of cross-platform not exactly a moot point, but less of an issue.
c) If you have the money, Conitec would be happy to do whatever you want... they're whores that way.
But it would cost you about the same as investing in a cross-platform engine to begin with.
Here is the really, really abriged version of the answer: not going to happen.