Ok, don't worry about those other replies, I think they just misunderstood
What you need to do is add your own line into the bottom of the "appearance.2da" and have the values for the skin match the name of your own skin. So find the mandalorian line, ex. "N_Mandalorian01", copy it, and paste it at the end of the file. Then like I said, change the texture values to match your very own file. Then you're going to need to make your .uti file refer to your new .2da line by adding a disguise property.
I know it must sound confusing (it does to everyone their first time) but it's a pretty straightforward process. I'd suggest using the handy search tool here and search stuff like "adding 2da line", "new mandalorian appearance", etc. You'll find your answer if you look in the right places. Good luck!