I do this for challenges (aka assignments) that are presented in categories that are read-only (everyone can “see”, admin can create/reply/see). Students are instructed to reply-as-linked-topic and post their responses in a different category. It works pretty well, though the reply-as-linked-topic button is hidden if your mouse isn’t on the message, which some people find confusing.
This solution has each student create a new topic for their solution. If what you want is a single conversation about the read-only message, then what you might want to do is reply-as-linked-topic yourself and put that new topic into a category that is see/reply for everyone.