Those "codes" are called Dvars. To host an XP lobby you would need a modified .self of the game with dvars values changed.
You would need some knowledge in PowerPC, but if you don't you can use a .self generator, choosing all the modifies you would like to have.

you need a jailbroken ps3 to host xp and if you do have 1 i think you should use choco's eboot

or just use eboot builder :/