Well, as I had said, so far the only file type I found that worked without registering was the .slp (PRO/Engineer Render). But I was also trying purely what both Softimage and Blender could support, and I stopped on that list when I found one that worked (in this case, with Blender.)
Blender, btw, is a free modeling program. How good it is exactly, I'm not sure. I haven't really tried to figure out how to truly use it.
In anycase, as for how to do it. You start with just opening the .mesh with 3DOC, and saving it as an .slp.
Then start up Blender, import the .slp in, and export as an .xsi.
That you can just import into Softimage no problem and play with it from there. Just be sure to get rid of the extra cameras/grid/box/rootscene stuff, since the export from Blender is the whole scene, not just the model. (Might be able to stop that, but I haven't bothered to try, it's really easy to select and delete a few useless objects.)
After that is the long drawn out process of doing whatever it is you want to do with the model. (I don't advise using this to resize. The UV map is gone, as are all the points the game needs for weapons/abilities/etc. You have to create all of those. There's also some little program someone wrote on the forums somewhere that can do resizes easily for you instead.)
When done with everything for that (check the guide in forge tools if you're not sure all the steps you do need for a 'new' model), just export from Softimage as an .xsi again, run the .xsi through the mesh converter from the Forge Tools, and go about all the stuff needed to add a new object of any sort to Sins.
There may be other ways of doing all this, with fewer steps, and less loss of additional info (the uv/points), but this just happens to be an entirely free method that I've found works. If anyone finds another way, I'd certainly suggest tossing it out for others to learn.
