There is no real in game or even ultimania books that detail any of the later members. The closest thing we get is that Xigbar mentions he was the one who found Marluxia in KH2:FM but gives no details whatsoever about where he met him. Even 358/2 dropped the ball and never really spoke of anyone's past beyond a few subtle hints here and there and even then most of that stuff was expanded upon in BBS.

The best we can hope for is either Nomura releasing a source book giving the backgrounds for them or maybe getting some more info in the next major KH installment that takes place after KH:coded which will either be KH3D or KH3.