... but if you want to be able to use the models in Sins they MUST be finished in XSI. In accordance with the modding documentation.
Not fully true... by example, you can use Milkshape for create the weapons point, the center point, truster, etc... and export the result to XSI file...I have use Milkshape for example because it support a lot of ( game ) format... other software are able to export to XSI format...
What you must use is the XSI to mesh convector, who from Stardock...
For modify, to my knowledge, only 3DMax have a plug-in for import .mesh file...