Ok,
I am going to try the sketchup thing too, but I may have more insight for you guys.
Sketchup pro only can save in XSI, that's true.
Now, I believe we have to load it into MOD TOOLS XSI, and then do the thing we would normally have to do to get it up to SINS standards (the developers wrote a HUGE tutorial on how to use XSI)
Then, we can save the textures as DDS files, which can them be editied in Photoshop.
(yes, another program)
Anyways, this is how I am going to do this. It may work, it may not. But I messed around with XSI a little bit, and it as well as the XSI converter and SINS itself, is very picky about what files get exported, in what format, and where.
Now to answer the question you are know thinking (what do I have to do to get the model up to SINS specs?) Here's what you do:
First, read that tutorial (found under the downloads section of this site)
Then, go into XSI with your model, and do what they tell you. But the basics are, you have to add the weapon points, and the engine points too.
If you don't do this, I am pretty sure that the model will not work.
Anyways, this is all an educated guess really based on what I have read and what I know. I do believe that if we work together, we can use sketchup, which seems very easy to learn. The only other thing I can say is, try it and see what happens!
DANMAN