Exchange mail, How can I save all emails from an account in Mac mail, then disable the account?

I have an Exchange account from an old job that been in my Mac mail. I have the email in offline mode (since I no longer have the email access). By keeping it in offline, I have maintained all the emails, but ideally I want to disable the account while still keeping the emails in a backup, on the hard drive, or in an archive (there is no way to reactivate the account at this stage). If I simply disable the account, looks like that would immediately remove them from my mail permanently with no way to recover. Looking for a way to save these emails. If I move them to "on my Mac" does that make a copy of the emails on the hard drive that is then separate of the server? If so, seems this might be tedious, not batched, but either done by folders or individual emails. Any help here would be appreciated. I tried the "export mail" but this process is less clear yet. I tried one folder and it saved into my Finder, not Mail, so not in any type of a useable, viewable archive. If there is a way to import them back into my mail, I can't find it. Any specific help would be appreciated.