In the scenario that you describe you only really have one option because of the lack of a Trust relationship. You will need to create new archives for the users in the new domain. Without a trust relationship you can do a standard domain migration so you'll probably be looking at importing and exporting PSTs for the mailbox (unless I'm missing something). If this is the case you may as well export the user archives into the user mailbox and then dump it all to a PST for import into the new archive in the target environment.