2
I have no doubt it *could* be done with some work. However, there was a very diliberate move to support MySQL by the XOOPS developers as (a) it open source and free to use, and (b) it can be used on just about any platform, where as MS SQL is limited to Microsoft platforms. If the changes aren't too great you might be able to get someone to make some changes for you for a price, but by doing that you will also complicate you upgrade path. In reality I would say you'd be better off setting up a second box.