Different Rules for a series?

You said they aren't accepting unsolicited MS, but that's not the same as not accepting queries. Have you checked to make sure they don't accept queries as well?

Hmm. I don't actually know much about how to go about trying to get in on writing for an established series. I'd suspect that if you were able to get an agent interested, that would help-- but I don't know how these things work. Do you submit a proposal? And if that's the case, how do you get an agent with just a proposal?

Sorry, just my not very helpful train of thought, there. Hopefully someone with more knowledge will see this.

If it says preferred -- but doesn't say that they absolutely only take agented submissions -- I would just go ahead and submit a query and sample chapters if I were you. Mention any previous publications, do a fantastic job on the query...and you'll have a better chance of getting a thorough read by the series' editors than you'll have of finding an agent interested in representing you for your query.

Don't worry about the agents other authors used. The reason the publisher prefers agented submissions is that they want to see polished work from professional writers...and writers can only get an agent if they meet those criteria. If they're even a little bit willing to look at work the author submits, and you can do a killer job on your query/proposal, you're far better off just submitting.