The matter of modelling and UV texturing existing meshes/textures is an on-going problem in Sins. Simply put, there is no easy way to convert the existing .mesh files into XSI Mod Tool or Blender: the ConvertXSI tool only converts from XSI back to .mesh.
As you mention if you have 3DS Max there is a MaxScript written for Sins can help you there but of course 3DS Max costs money.
When I was running XP I used 3DS Max and Adobe Photoshop (with the Nvidia DDS plugin) to do modelling and UV texturing tweaking (I say tweaking because I'm too lazy to start a ship from scratch when other people have done most of the detailed work (e.g. EvilJedi, 7DS team).
Now that I'm using (and loving) Windows 7 I want to start learning the free XSI Mod tool from Autocad/Softimage (because 3DS Max is unstable in Vista/Windows 7).
I've tried converting .mesh files using 3D Object Convertor (you can Google for it) and the unregistered version supports .IGES format (put the .mesh and textures in the same folder and drag it into the 3DO window). It imports successfully into XSI Mod Tool but randomly crashes after a few minutes or if you try to save in XSI format.
I'm going to look around for some other convert programs to try and get .mesh importable into XSI over the weekend... if anybody has a successful way to do this please let us know!