I know it's possible to do things with users who have never interacted with your community, though I am not sure what they used to do it. No client I have used even has the basic moderation functions down very well, and through the website interface I have only seen mod actions appear within posts in communities I mod and nowhere else.

It depends on the app or interface you are using. In Mlem there is an "Add Moderator" button in the moderation tab. I believe in the web interface it is easiest if someone posts, or comments, then in the three dot menu there is an option to make moderator.