When I finally migrated from Eudora to Mail a few years ago, I imported all my email archive (going back to 1995). I can’t remember the exact process now, but it worked very well and I now have all my email in one place and easy to search for the first time. Highly recommended!
I’m sure I used @tonya’s excellent advice, and definitely used the equally excellent Eudora Mailbox Cleaner. Note the link in Tonya’s article is out of date, but the one I gave works. This is an amazing, invaluable utility when migrating.