I don't know of any plugin that is already built to do what you want. Like most of the rest of us, you will likely have to write some Node JavaScript to create the plugin to suit your needs. I don't have the time to write custom code for free, although if it is worth money to you, the NodeBB development team may be interested in consulting for you.
Now that I re-read what you originally asked, I'm curious if you actually need "Sockets" (as in SocketIO, which is server-initiated TCP/IP connection to a browser) or if APIs on both the hosted NodeBB server and the localhost server would do this better.
Either way, if you choose to go the NodeBB plugin route, you will need to know which Hooks are available. A "hook" is an event that your plugin subscribes to. When that event happens in the core NodeBB server, it will call the function in your plugin which subscribes to the event.