Hi Heiko! It sounds like that you’d like to do doesn’t really require Keyring per se, since Keyring’s main purpose is handling authenticated requests (tokens, signatures, custom headers, etc). If you’re requesting publicly-available RSS feeds or CSVs, then all you really need is what I’ve got in the Keyring Social Importers plugin, or you could take a completely separate (but similar) approach since it’s just making requests and then parsing the response.
That being said, the approach you’re describing above would also work I believe (make a Service + Importer combination for RSS), you’d just need to make the service something like the HTTP Basic one, but without any of the pieces related to sending a user/pass combination (so it’d basically just end up being a “pass through” Service, that makes requests without any additional authentication required).
Here is an example of someone else who wrote a custom importer, so you should be able to see how/where they put their files (basically you can put them anywhere, and then hook into `init` to load the code properly): https://github.com/cfinke/Keyring-Facebook-Importer
You should be able to take a similar approach with the files for Services + Importers.
Hope that helps!