I have read that it is possible using imapsync or Thunderbird to
migrate mail from another IMAP server into Zimbra. I wanted to
mention a method I tried that failed, but seemingly should work.
I set up the ZImbra webmail client to talk with my primary IMAP server
and then used drag and drop to move the email into my Zimbra mailbox.
It copied the mail but I lost the time/datestamps of the original mail.
This was using ZCS-5-RC2.