There's a complete section in the wiki about
migration.
However, if they can export their address book data in .csv format they can be imported into individual accounts. If they can connect to their current server with IMAP then IMAPSYNC will do what you need (detailed in the wiki). For the user account names then, again, if you can export that you can script their creating in ZImbra. Read the wiki and post again if you have questions about it.