For the third game at one time you could possibly get access to the source code, might still be able to. I doubt that's what you had in mind though. Other than that it's conceivable that if you could modify the XCR files you could create stuff but I'm inclined to say even then it's not likely. The unofficial wiki had some downloads to edit XCRs I think, I don't really know what one can do with them though. If you're determined to try I'd look around some fan sites and see if anyone can tell you more about modding it and such. Back when community sites were active for the first game I heard talk of mods as well but I never really looked into them.