Just thinking and guessing a bit about this:
I would think that the facebook account type, is just a regular jabber account type, with a different icon, and @chat.facebook.com pre-filled when you create the account.

So, wouldn't it be possible to find the configuration file in which the IM-accounts are defined and simply modify the account-type inside that file?
Does anybody know where the IM-account configurations are stored? I can't seem to find it...