Based on the complexity requirements alone, this isn’t something easily done. Though depending on the version of FOG you’re using, you could create a crontab that could lookup all your hosts, see which are set to join the domain, and add any snapins you want automatically. You can even use the same principle during a deploy task through the use of postdownload scripts. Check if ad is to be used, if so, set the snapin for that host, and schedule that snapin task, then complete.
Of course i’m talking complex stuff to think about. But ultimately it’s not impossible, but it’s not automated in fog. Complexity level, i think, would be around a 6 or 7 on a 10 base scale meaning it’s not simple by any means, but it’s also not too hard. There’s some information on the use of the api system (assuming you’re using a version of fog with the api stuff) floating around the forums.