What you want is perfectly possible, there’s an API for creating and editing posts: it just hasn’t been done yet.
What’s the use case for having one post appear on multiple sites? If it’s for resilience, surely that would be better achieved using a CDN instead of unnecessarily duplicating the data? If it’s to combine multiple forums into one central superforum, again that can be better achieved without duplicating posts. I’m too young to have used NNTP or newsgroups, so I’m unfamiliar with how those worked. 
And what is fed.soc.net?
I can see the value of integrating with other services, I can imagine a plugin which syncs posts between a Discourse category and a Facebook group being used, for example. I think it has been requested before. But you would only need to duplicate the data if you do not have control of the other platform - imo duplicating content introduces a point of failure you don’t need to have
The federation feature I think is worth pursuing is federated profiles - there’s some discussion on that here: