In media section of this website there is a download called "Forge Tools". Most of it is out of date, but the documentation (mostly) and three programs are still good. Those programs are the GalaxyForge, ParticleForge, and ConvertXSI. The latter will allow you to convert .xsi files into .mesh, but not the other way, so you can't edit the meshes already in Sins.
That said, myfist has a script for 3DS Max that can turn .mesh into .xsi, but you'll obviously need that program. He has some Sins models (mainly capitalships) for download in .xsi form here.
