[Dovecot] convert to mdbox

Hi

How can I convert all virtual mailboxes from maildir to mdbox? Manually, one by one, working, but I have a lot ...

Virtual users and domains stored in sql database.

Thats what I want:

mail_location = maildir:/var/vmail/%d/%n/Maildir to mail_location = mdbox:/var/vmail/%d/%n/mdbox

Thanks Levi