I went through the same thing recently...
I pulled over all their files from /opt/zimbra/store (storing them in say /tmp/backup/). Then I did (as user zimbra):
% zmmailbox -z
adminAuthenticate
admin@acme.com my_password
sm deleted_user
cf /Recover
addMessage /Recover /tmp/backup/msg/0 /tmp/backup/msg/1 <etc>
exit
I still need to figure out how to copy over the calendar though. If anyone knows how to pull over the calendar please let me know!!