Migration from courier-imap to dovecot

[alex]

hello,

I search a solution to migrate a courier-impa mail accounts in maildir 
format to a dovecot mail accounts in sandbox format.
The first server is an ubuntu 10.4 with courier-imap in maildir format, 
the accounts are managed by postfix 2.7.0 with MySQL
The second server is an debian 7.8 (wheezy) with dovecot 2.2.13 in sdbox 
format, the accounts are managed by postfix 2.9.6 with MySQL

My request is, how to import and convert the mail accounts ??

cordially,

*--*
*Alexandre Bagouet*