After much Google research, it seems that the only way might be to do, as follows:
Open Mail > Windows > Previous Recipients. From there, use Command + A to select All of them.
Then choose "Add to Contacts" but create a new "Contacts" to add to. Once there, you can export them as CSV or other formats to wherever you want.
Caveat: I have not tried this myself. This suggestion arises from 20 minutes of Google research. Caveat Emptor, okay?
Ian