Restore email from Time Machine

What is the recommended procedure for restoring old email messages from a Time Machine snapshot? I had to do a clean install of Big Sur on this computer and I did not restore all my old email right away because it was going to take a long time. Now I have lots of new email stored in my Mail app, and I want to retrieve the old messages without deleting the new ones. I understand if I use the Time Machine restore function to load the email state from three months ago, it won't keep the new email.

I should also mention that this is email from Earthlink that is not stored on their servers but was previously downloaded to my MacBook.

Thanks.