Well so far the only models we can get in the game a simple ones (thanks to cchargin) (
http://www.lucasforums.com/showthread.php?s=&threadid=129555) like blasters, masks, vibro swords, Basicaly anything small and without animations.  
 A good way to start would be to take one of KotOR's exsisting weapons or items and reskin it and make it a unique item. There is more than enough info to help you get started. 
 Also you should get Kotor tool and read this Intro to modding KotOR with kotor tool. (
http://www.map-review.com/index.php?page=tutorial_view&id=28) 
 After that you should read 
 Guide for the newbie: how to install mods. /What tools do I need to start modding? (
http://www.lucasforums.com/showthread.php?threadid=129789) and for more secifics check out this on modding KotOR check out this thread Do you Want to mod SW-KotOR? Then Start here. (
http://www.lucasforums.com/showthread.php?threadid=124284)