I don't think there's a fixed policy on how we'd add new admins.
I can imagine us promoting someone to forum admin when we generally run low on active admins, such as due to an admin (who actually used to be a regular contributor) stepping down, or increased maintenance work load.
If this were to happen the current forum admins would probably reach out to regular contributors (to the game source code and assets, the forums, IRC, server management and other places / services) asking them whether they would like to help administrate the forum.