Thanks for reading - I'm not necessarily attached to using the "archive" function in Apple Mail: the important thing for me is to keep ancient emails on an external drive, so that if I want to read them back I may do so, but not take up lots of space on my main HD.
The archive function seems to compress them, but keep them "locally."
I don't want to do anything involving codes & commands: I just want to store my stuff externally, and retrievably.