By the way, this seems very much like a "Pro" feature. I know I'd be willing to pay $30 for an update that could handle this.

Exactly, I want to be able to segment my mailing list. It's very important for being able to deliver the highest value messages. Thanks for the response.

You need to have a group or interest field where to store this data, don't you? with that said - currenty it is not possible. We plan to add a mapping in a future release. Where people could determine the joomla field and the mailchimp fields. So the user won't have to fill more data than necessary.