sorry i forget what stuff is NE only commands. this can be done it'd just take a bit of scripting on your part. the first step would be get the multi-mailbox server setup done. Then you'll want to go into your cos settings and limit the server pool to the new server only so that way when you create users they're never created on hte first server.
Then the logic would be this
1. export the user's account (look at the blog post phoenix posted)
2. delete the user from zimbra
3. re-add the user to zimbra (should get created on the 2nd mailbox server now)
4. import the user's data
there may be a way to do steps 2 & 3 better, but i'm just not sure what all you would have to update for the user like zimbraMailHost and all that to point ot the new server.
Of course if you try to script this using looping through zmprov getAllAccounts it would be very worrisome cause if something goes wrong with the export and you delete teh account it'd be very bad.
What are you trying to accomplish with this anyway? Are you just trying to have the mta and mailbox be different servers. If so, it's a lot easier to split off the mta portion to another server. Or what you could do is move the entire mailbox/mta install of this server to another server, then split off mta to teh old server when you're done. Look at this page
Ajcody-Notes-Server-Move - Zimbra :: Wiki