I don't think that the game/Sony lets us do that. You can add friends. But, I don't think there is any way to add everyone from a clan. In fact, the only thing that even relates to a clan that I've found in the game is that you can list your clan name under your PSN or GT name.

Best way I've found is to go to http://www.dadsofdestiny.net/members/ and select a few criteria (console, game time, mic, etc.). It narrows it down a bit. When you get a list of hits, it'll still be something like 200+. Most of the more active posters (I think) have changed their avatar picture. Copy/Paste their names to another document, make a list, then "Add Friends" like a madman next time you get in the game. If you have a bluetooth keyboard ($25 Logitech thing from Best Buy works fine), you can add like 30 in 15 minutes. Be sure to put "DoD" in the notes.