Hi,
I'm gonna help you and point you in the right direction, but before I do I feel compelled to say something even though it might be a bit unnecessay: You obviously
already know that someone else put it together using the component parts and you say
Wouldn't a better first step be to read one of the many guides and tutorials here? You skipping that step and going straight to posting a mod request could in my opinion very easily be interpreted as "My time is so much more important than everybody else's that they should do a modding project so that I don't have to learn this stuff."
Given how very polite your post is I know that's not what you meant but reading the guides first and asking for help second might have been a better move.
With that off of my chest: You need a mod called
moreclothing. It allows you to add costume mods to the in-game menus and gives you all sorts of customization options. Get that. It requires the Loader. If you don't have the Loader just get
sby loaderpack which comes with moreclothing preinstalled. When adding stuff to moreclothing you'll also want
Show Mod Info since you may want to add only some parts of a costume mod. Show Mod Info will tell you what's on the menu. You'll get it when you get started with moreclothing.
Hope you didn't take any of that as being more harsh than it was intended. Welcome to the forum!