I'm confused about what exactly you're trying to do (and why). Can you clarify?

Only personal and OpenID accounts can join or subscribe to a community.

Communities can't subscribe to anything. Their "reading pages" consist of entries made by members of the community.

Feed accounts don't interact with the site at all except to be subscribed to by personal or OpenID accounts. They only mirror articles from the source blog for two weeks, as a convenience for people who don't care to use an RSS reader.