A quicker way than creating aliases for all the users would be to create a domain alias, per the instructions at:
http://wiki.zimbra.com/index.php?title=ManagingDomains
Basically, have both domains on the Zimbra host. Batch rename all of the accounts as mentioned above so they exist in the new domain. Delete the old domain and create the domain alias. That way, existing users and new users alike automatically receive mail for both domains and it's a quick deletion later if you choose.