Use a hex editor like
HxD and open up the character's CSSSlotXX.dat, this file contains the information about the clone's costumes that are selectable in the CSS. Starting from offset 0x20 (The start of the third row in 16-byte view), you'll see the bytes that represent each individual costume.
For example, this is from Mario's CSSSlot:
"00 00 09 06 01 03 ... 01 0A 0C 00"
It's in hexadecimal, so 10 would be 0A and 11 would be 0B, 12 would be 0C and so on.
Each costume is made of two bytes, so you can imagine a comma in between both costumes like this:
"
00 00,
09 06,
01 03, ...
01 0A,
0C 00"
The
first byte represents the color of the costume, but it serves no real purpose in the game. I'd keep the red/blue/green costumes at 00/01/03, otherwise just do what's convenient for you.
The
second byte is the model associated with that costume. For example, 00 is Mario's default clothes and 06 is his fire-flower outfit.
"0C 00" tells the game that there are no more costumes to load.
The order the pairs appear in this file will be the order the costumes appear in the game.
With the above in mind, we can add more costumes. Move "0C 00" and move it a few of pairs ahead depending on the number of costumes you want to add and fill in each pair accordingly. So, for example, if we wanted to add 2 costumes (11 and 12) to Mario (more on that later...) we'd go from "...01 0A, 0C 00" to "...01 0A, XX 0B, XX 0C, 0C 00"*. Save and the file should be good to go.
A few things to note, however:
1. Make sure you've added the cosmetics (CSP, BP and Stock Icons) for each costume in their respective places. The game WILL crash if any are missing. I don't think Costume Manager works with PMEx very well so you're going to have to use BrawlBox to add them yourself. If you aren't sure how to do that, you can ask me here later.
2. The added costume ID's should be enabled in the FighterXX.dat. If you haven't already, use Brawl Config Utility and go do that.
3. Unless you've given a character two cosmetic slots (Like Wario), a character cannot have more than 10 costumes.
4. Otherwise, in BrawlEx, a character cannot have more than 12 costumes.
I hope that this was helpful.