Advice on merging 2 groupwise systems

I've searched the forums but since each situation is slightly different, so I figured I would start a new post.

I support two different school districts, each with their own eDirectory tree and Groupwise server(single GW server, single domain/PO/GWIA/WebAcc). For economic reasons, these two school districts are merging at the end of this year, and will become a single school district. My goal is to merge the two Groupwise systems so that in the end, District A and District B both exist in then same Groupwise system, with a single domain/PO and all mail, contacts, calendar items from their old systems exist in the merged system. They will most likely have a new domain name for the new(merged) district.

Existing domains:
mail@school1csd.org
mail@school2csd.org

New domain:
mail@school3csd.org

What would be the best way to accomplish this goal?

Is it possible to maintain the existing e-mail addresses in addition to the newly created "mail@school3csd.org"?

Any further information needed?