You're ih luck. This request is pretty simple to do, it has tutorials already written, and you seem willing to try give it a try with some help
This link should help you with editing the robe properties.
http://www.lucasforums.com/showthrea...hreadid=132253
And this one will help you figure out how to make Atris body model (her robes) available to other players.
http://www.lucasforums.com/showthrea...hreadid=122003
Both of these resources were created for KotOR, however they (should) work just fine with TSL. Feel free to post questions if you need a point clarified or if you hit a snag.