I tried to add the forums RSS and it fails to add it. Works perfect with Wordpress or other popular sites that provide a RSS feed. That would be awesome to have the RSS adjusted to be recognized in Currents, is an awesome application.
Even if it was working, since the XF feeds don't even contain as much as a snippet of the post as the item description then the feed is pretty pointless for use with Currents, and other aggregators, because there would be no content available, just the thread title.