I haven’t used it yet, but I saw there is a RSS Polling plugin. So my first strategy would be to use that or a modified version that also makes API calls in addition to the RSS feeds.
Or build a standalone service that lives outside Discourse that fetches posts and republishes them via the Discourse API. Or maybe a third-party service like IFTTT could fetch posts and republish them to discourse via email without needing any other glue.