User Scripts for Mail-In-A-Box allows the management of MIAB via CLI.
With this script you can copy users from Domain -> Domain by parsing current users for a given domain, and then copying them to the new domain.
You can also archive whole domains as well.
See the readme for more details.
I plan to add more to this as it grows.
Screenshot of output(Example given is a copy from example.net to example.org):
Thanks @alento & @HoustonBox for the idea!