Moving mail from permanently offline account's inbox (macOS Mail.app)?

I have an old, no longer accessible IMAP account (or two) with mail that I need off of it. I can access the mail in this defunct account (cached/local copies now on my machine); but my attempts to move the emails in the inbox, or even copy them, fail. Is there some other way to export them to a mailbox on my hard drive (within Mail.app)?