I think that it would be interesting to have IMAPSYNC installed with a GUI.
You can provide Hestia with a csv file with:
mail.server.com;[email protected];p4ssw0rd;
mail.server2.com;[email protected];p4ssw0rd;
And then feed that to imapsync.
Hestia could then create the accounts with the same passwords, assign the username = domain.com and a default plan.
And then migrate all the mails in the mailboxes.