Suppose I want every starred message in a specified chatroom to be posted in my chatroom. How should I add the feed in my chatroom?
You can't - the only sorts of feeds enabled by default on chat is RSS. We use it to feed external news sources and SE posts into chat as needed.
Chat itself has no RSS feed mechanism (and if you're going to go so far as to write a bot to scrape chat to convert it to RSS, might as well go the whole hog and build a bridge).
So simple answer is there's no way to do what what you want to do easily.