I'm not sure what your accounts are stored in now, but assuming that you can dump them to a text file, you can massage the file so that each line will contain:
Minimum:
ca
account@domain.com 'password'
Optionally:
ca
account@domain.com 'password' cn "firstname lastname" givenName "firstname" sn "lastname"
If you want aliases, on later lines:
aaa
account@domain.com alias@domain.com
so, you end up with a file like
ca
user1@domain.com user1password
ca
user2@domain.com user2password cn "user too" givenName "user" cn "too"
aaa
user2@domain.com usertwo@domain.com
etc...
su - zimbra
zmprov < filename
This will provision all your accounts.