Mail can't import mbox files from a Mojave disc into a Monterey disc.

I'm not sure that the incompatible systems are the problem, but: I'm trying to retrieve the mailboxes in Mail on my Mojave-run Mac into the Apple Mail app on my newer, Monterey-run Mac. So far, nothing has worked. The official Apple How-To procedure goes into some detail, but none of it works for me. What I'm working with are a Mac M1 Mini, which now has the 1TBssd boot drive from the Mojave-run Mac plugged into it as an external drive. The drive has my User files, applications, and system files, and it was the boot drive for the previous computer, so I was hoping that, as a last resort, I could restart from the Mojave drive and export the mail files to the newer disc. Nope. No can do. So is there a way to place a Monterey OS on a formerly Mojave drive without wiping out everything? or is there a better way to get the job done?