The "blog" service only asks for the blog url and does not even mention RSS (we look for the appropriate link tag on the page). I've also just added the feature that Bruce requested, so you should be able to add a "Add this blog to FriendFeed" link to all of your blogs which prefills the url box and directs them back to your site after it is added on FriendFeed. Let me know if you encounter any problems.