All this:

If you’re comfortable with Python, you can try this out: Lemmy.py

I don’t think there’s a way of doing this automatically at the moment.

With Lemmy’s API you could set up a bot that takes care of listing all the local communities on a remote server and then searching them from yours, which would make them appear in your search results.

But if you want the publications of this remote community to federate with your server, you need to have a local user subscribed to it.