[Simon Lloyd]

Generating a key at a user request and storing it against their name would be fairly simple, you issue would come when you firstly want to check from your gaming software for the validity of a key generated in the forum, check that the users are the same (i.e no key sharing), check that it is valid (so checking if there are time or date constraints (i.e is it an expiring key)......etc

Anyway, you could create a plugin for a button in the users profile to create a random number and insert it in to a new column in the user table, a way of generating a unique key would be to use:
$key = md5(microtime().rand());