[Dovecot] Capturing credentials for imap sync

[Master Abi]

Hi List

All the imap sync apps I could find requires the username /password 
credentails to be known before a sync occurs. I have Dovecot using ldap 
acting as a nearline backup mail server to MS Exchange. Every hour 
imapsync syncs mail between Exchange and Dovecot. This all works fine 
becuase the users credentials are known, but when new users are added I 
would like the process to work seemless like this:

The user is added to the Active Directory. The mail client is configured 
with 2 accounts with identical credentials, one of which is connected to 
Dovecot. When the new client connects to dovecot, the credentials are 
captured and stored so that imapsync can use it. This

If there is another way this can be done with dovecot, could I please be 
enlighten. I have read the doco on migration but I am not quite 
migrating but more replicating.

Thank you