Short version of the answer: Copy the imperial AT-AT entry in GROUNDVEHICLES, paste it into the same file, give it a new name and change tag to Rebel. Copy the AT-AT entry in GROUNDCOMPANIESEMPIRE and paste into GROUNDCOMPANIESREBEL, change the name to something new, change affiliation, change the unit which makes up the company to the Rebel AT-AT you created in GROUNDVEHICLES instead of the imperial one. That should do it, or at least get you started. There is no guide to modding this game. Everyone who has modded anything has looked at the XML files, figured out how they interact by trial and error, and slowly learned how to make changes that work. Until there is an SDK, anyone who want to make a new mod should do the same. Even armed with the short instructions above, you're going to have to dig through the code and learn what the different attributes do by trial and error. There are no shortcuts to modding.