Finally, I found out that (on EC2 environment) it's easier to install another zimbra server and then perform user migration from old-server to new-server by command utility:
/opt/zimbra/bin/zmmailbox -z -m $USERNAME getRestURL "//?fmt=tgz" > /backup/accounts/$USERNAME.tgz
Well, this only works for a small number of user, but at least it works for me
however, zmmailbox doesn't export password does it???
I successfully exported everything, but password. I ended up changing password on the new-server to log in (per account).
Any more hint about migrating password?