Recovering Apple Mail emails from a backup file on a new MacBook Pro

Hi. I had a series of disasters. The email server company I was using had a server failure and I lost all the IMAP emails stored there. Fortunately they were all stored on my MacBook Pro 1708 Apple email. Also fortunately I had made a backup copy of the inbox.mbox folder. I have no Time Machine backup. Then I accidentally dropped that computer and broke the display. I was unable to get an external monitor to work. I tried the headless Mac trick but could not get that to work either. The one good thing is long ago I had to install a removable NVMe SSD. So I removed it, bought an external NVMe SSD external drive adapter, bought another MacBook Pro A1708 and I can now read the old SSD and I retrieved the old inbox.mbox file which I assume contains all may old emails. But on my new computer with fresh install of macOS (Ventura) there are no Apple Mail accounts set up and therefore The FILE-IMPORT option is greyed out. All FILE options are greyed out. I can't add the original email account because that account no longer exists. How can I get my old emails back?

[Re-Titled by Moderator]

Original Title: Transferring apple mail