Greetings Odhiambo, thanks for your response. I personally don't use Postorius, and I'm not even sure anyone on our team is using it. A quick search of all our repositories shows only a single mention, back in 2017. A member of my team uses hand-written Python scripts to do much of the configuration, which is why I'm slowly trying to convert everything over to using the REST API.
As to your comment about not understanding my question. I basically want to add a domain to our Mailman installation, or to set a different primary for different environments, so that I can create mailing lists against those other domains.
mydomain.com <- primary in production and local development staging.mydomain.dev <- I want this to become primary in our staging environment.