Instead of importing user accounts, couldn't you just move the entire server over?
If so this might help, it works for me:
Moving ZCS to Another Server » Zimbra :: Blog
This might also be worth a look:
http://wiki.zimbra.com/index.php?tit..._to_New_Server
If the new machine is going to have a different hostname in DNS, then you will have to trick the zimbra installer by setting up another DNS server (probably on the same machine) that points the old hostname to the new IP address, and then have your new machine use that as its DNS server.
After that, there must be a way of changing the hostname to what you really want (without making Zimbra freak out) but I haven't personally had a need to do so.