Making a custom .utc isn't hard -- the easiest method is to use KotOR tool and start by modifying an existing .utc. I recommend modifying the Templateresref and the Tag field to a unique name but use the same for both. This will help you stay sane.
Getting the .utc into the game is not as easy. I recommend the scripting method (
http://www.lucasforums.com/showthread.php?s=&threadid=143536) unless you are planning to make a whole new area..
Here's an old thread for your amusement to show you that even Darth333 had to learn this once.
http://www.lucasforums.com/showthread.php?s=&threadid=126311)