[Dovecot] mbox with sub-folders and messages?

Hi,

Is it possible to have dovecot configured to use mbox format and yet have folders that contains sub-folders and messages? I'm thinking of a simple setup where any files directory with extension .sbd are presented to the IMAP client without the extension. So if you have a mbox file named BOX01 and a directory name BOX01.sdb would show has one on the client side (note: that is the way Thunderbird keeps it's mail locally).

I know that the Maildir format does the sub-folders+messages, but I'm not really interested by the one file per email.

Richard