I haven't found this feature anywhere. It's often used on reddit to make sure a post's content doesn't disappear if a subreddit gets removed or is shut down for some reason. (edit: or recently wasn't accessible because a subreddit went private in protest or otherwise. Or, more normally, you want a single content page to share in multiple communities for easier edits to one source, etc.) You just post a link to the original. The only thing I can think of to do it here (or on a different instance) is create a community that only myself can post to as a mod, but unsure if such a thing is frowned upon.