There is a way to do this, although it's slightly fiddly at the moment. You need to publish your custom feed data to a file, then add that file as a source and append the data from the source and the original feed, removing any duplicate records before publishing. You will also need to turn on auto-publishing in the publish block, so this feature can only be used in Omniscope Enterprise.
We may introduce a simpler way to achieve this in the future.
Attached is an example of this using the rss feed provided.