How can I add some custom Naga Sadow robes I've made to
 where you get them in-game in Naga Sadow's tomb, when
 you get the Sith lightsaber? 
 I still want the Sith lightsaber, but how would I add
 my new robes to that statue's inventory? 
 (without screwing my game up, I mean)Open up k39_sabersarc.utp using KT, add your item to the inventory, and save to your override folder.  
 If you need a tutorial, you can find one here (
http://www.lucasforums.com/showthread.php?t=134067)