Asimov, its a piece of cake.
At least it is with the right tools. Why you'd want to add a controller that bigger than the console I'll never know but it can be done.
You just hook up switches between the GBA's. I think you could even wire it in parrallel so you can still use the original controlls. I'm not an electrician so....
But I try neways. But you know what? You're better off using the GBA player in conjunction with the soul calibur 2 joystick. Should work fine. Also look at the digital controller
made specifically for the GBA player.