I have created some software that is capable of synchronising posts from Reddit to Lemmy. It's still a little rough around the edges, but it works as a such:
People can request new subreddits to be mirrored on !requests@lemmit.online. A bot (open source) will monitor the threads there, and if it finds a new request for a subreddit, it will make a new community on the Lemmit server, and add it to its monitored list.
It will then make periodic checks to see if any new posts (it doesn't copy any comments) have been posted on reddit, and copy those over.
Users can then subscribe to those communities from their own lemmy instance, and from there federation will pick it up. Or at least, that's the theory. At the moment, federation is not working awesomely, and that is where my lack of fediverse knowledge comes in. Maybe it needs more time, or something is not so properly - I don't know.
Furthermore: registrations on this server are closed. The point of this service is not to become a community on its own, but to deliver, ehh, "original" content to all the rest of the Fediverse while it's going through a ramp-up phase. Besides, the instance is running on a pretty small vps, and I rather have this thing manage itself. There is a !about@lemmit.online community for further questions about the project itself though, in case people want to discuss it further.
So ehm... Let me know what you think :)
Use the search function in your instance, and you'll likely have to click the 'search' button twice to bring it in. Put the original URL from the developer's instance in the search, like this:
https://lemmit.online/c/IdiotsInCars
Then you should get a link for it.
You get the 404 because your instance doesn't know about that community yet. It has to be made aware, and then connected to it via you doing the search.
Just tried and still doesn't work. Do you get any results when doing it that way?