Ah, but you don't add the Adcode like you would add a signature as a user, it's done in the templates. You can put what you like in there, as long as it conforms to the syntax of whatever forum software you are using. It's only possible for board owners and admins though, not users.
Who are you replying to please? It's possible (and quite easy via templates) to write code that allows the adsense to be shown a maximum of 3 times in sigs on any thread. You need to find a mod for the software of the forum you run, or write your own.
That was just my opinion for the OP. Anyways, If you can figure out how to keep it within rules then it must be given a try to check the results. Good Luck