archive cyrus mailboxes to .mbox

[Bettina Hafter]

hi there

our clients have huge mailboxes with huge folders and
10000 of mails. so I want them to archive their mails.

As I don't know every mailclient, I would like to offer
them that I archive their mails directly under
/var/spool/imap/user>. 

I was googeling around, but could not find anything
(tools or similar) to convert cyrus-mailboxes into .mbox
or .txt or similar (the other way around I could find).

Has anybody an idea?

bettina