We have one ZCS network edition server, with a number of users that are no longer with the company, but their e-mail must be maintained. Since I can find no way to delete a user account, but also maintain their e-mail store, is it possible to move users and their e-mail store from the network edition server to a new instance/server of the open source ZCS?

By removing the users from the network edition server, we do not have to regularly increase the number of licensed users and we do not have to continue to backup these users as part of the regular backups -- they only really need to be backed up once.