it's possible to use an external imap backend instead of the internal zimbra backend?

I want to use Zimbra (if it's posssible) to substitute a Horde Webmail. In this case, the Horde Webmail is just the front end to the user, and connect to the mailbox of the user using IMAP. It's possible to tell the Zimbra that the mailbox is in a external account?

Migrate the full mail service to Zimbra is not an option in this case. The SMTP/POP/IMAP backend must continue in the actual servers.

Best regards