You just add a custom field for users to enter their steamid with. You'll need to modify the code significantly if you're using the SVN version of ULX or if you're using SMF2.
Just using the non SVN and the SMF 1.1.11. This may sound like a noob question, but how do I add that custom field?
(Not a noob, I code PHP and stuff, just sick and not really thinking straight atm)