Yes, I'm not asking if it exists, I'm asking if it would be possible to make such a plugin that spontaneously opens a websocket to relays and slurps in some notes.
Discussion
Sorry, I don’t know enough about how Joplin is built to answer.
My guess is this would be non-trivial, as there currently are no social media or collaboration features.
Their dev community is active and responsive - I have opened issues and requested features in the past.