Mail: Merge mail account with new account or..?

I have made a big mistake changing my wife's gmail account to another email provider without importing her old emails to the new account. Now she is able to see her old mail in the old account in Mail, but she is not able to send mail from the new account in Mail because its the same user name.

So.. anyway I can merge or import the old mails (from Google Takeout) so that she can see the old mails in the new account in Mail? It would also be fine if she can keep the old account to read from and send and receive new mails from the new account