It should “just work”. It does “just work” for me. You simply reply to the email you receive on the topic from the same address. For example Save Apple Mail attachements to specific folder was posted by email (you can see the small email icon on the right corner of the message (well I can, I don’t know if it is globally visible)).
If you are having trouble replying via email, start a new topic on that subject in the meta category and include the details of what you are doing and what results you are getting.