Yes I know that, but I was just thinking about easy method to do what original poster asked about, i.e. make existing users to give explicit voluntary consent to process their data via clear affirmative action. Under GDPR you need to collect such consent from existing users and be able to demonstrate they gave it actively and when they gave it. I just thought that existing functionality could be used to collect that.
While it is easy to ask new users for consent at the sign up time by adding user field in the admin panel and making it required, Discourse has no functionality to collect required GDPR consent from existing users (except maybe asking everyone individually). As May 25 deadline approaches every working method (even if not perfect) would be good.