I know how to do it:
It would require a USB Key, A Cheat Code Device, and a USB program for PS2 (usually included in Cheat Code Devices).
You just use the disk to get a copy of the game's executable, from that you make a cheat code that saves the game memory during the part of the game with the costume you want while it's running to a memory card. Use the usb program to put the data on a usb key, then take it to a computer. Then do that same thing with a normal sora save file, and examine the data on a computer, if there is a part that is different, then THAT's what needs to change for the game to make you get a new costume (or, Potentially: Roxas&Riku).
Unfortunatly, I have no USB Key, and it isn't quite as easy as it sounds... it would take all night trying things. There are a lot of things that could be different between the games' data (The world your on, party data, Stats, etc. ) Litterally everything in the game.