You can setup a JSON file on your own server, and point the URL to this file in the additionalfeeds array. It can look like this:
https://foodl.org/extradiscofeed
Handling the combination of twitter, facebook etc altogether is not as streamlined in simplesamlphp as I would have liked it to be. But may be we can improve that in the future. Right now, I think you have to handle some login in the application about that, or possibly use the multiauth? module.
Here is documentation on the JSON format:
http://discojuice.org/docs/latest/discojuicejson
Andreas