I have been buzzing around here all day looking for help with my issues surrounding the zimbra data source capability and have finally found the answer so I thought I would share here.
I am migrating 1 million + mailboxes from qmail-ldap to zimbra and was needing a quicker migration plan than IMAP sync. So for what it is worth, my plan was to when migrating an account to the new zimbra platform, I would create the account and then add an external account - this being an account to the existing mail platform. Using the zimbraDataSource attributes, the account would (in my plan) migrate itself.

I had the problem where I did not know the users' password and qmail-ldap pop3d gave me hassles with authenticating by cleartext comparison. Courier-IMAP's authlib allows cleartext comparison, so using the pop3 server from courier-imap (as it is faster than IMAP and puts the mails in the migrated user's inbox) we are using the scheduler to connect back via pop3 to the legacy mail system and pop the mail off there and put into the inbox of the migrated mailbox on Zimbra.

We have yet to test this in terms of performance, but the early indications are that it is much more efficient than a perl script utilizing imapsync.

Anyway - thanks to the people that have given me assistance today and never told me to stop being silly!

If anyone wants to know anything further or has any tips/advice - I would appreciate it.