to convince people to migrate from X from Zimbra, it would be convenient to be able to let them use Zimbra while keeping their old mailbox (until the migration).
Zimbra would behave like a virtual mailserver, receving IMAP commands and forwarding them to the physical mailbox.

Is it possible today?
Is it planned?

A possible workaround is to have a daemon process sync the mailboxes between Zimbra and the physical mailbox. But this means the server is not virtual anymore.