As far as a file converter, I don't know of a standalone or web-based solution for the platforms you mention. I have been closely tied to this issue for over a year now, and found that file converters only exist for the higher-end CAD packages (ProE, CATIA, UG, I-DEAS). For others, the only feasible solution is neutral files (IGES or STEP), but unfortunately you are left with a 'dumb solid' which cannot be modified.
Recneps