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.
And this one will help you figure out how to make Atris body model (her robes) available to other players.
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.