Yeah, it is very simple. Just create a new forum and place your ad code inside the description. You can then set it either as a forum for that sponsor or just set it as a link only.
I haven't looked at that mod that Ozzy posted but the mod I think of which may be what he posted is to display a specific banner inside a forum which is also pretty cool. Otherwise this would have to be done using conditionals and a mod is always easier than that.
## - I see the mod that Ozzy listed but I haven't used it. It was not the one I mentioned but Ozzy's mods are always useful!