You can always add content, just drag and drop... big benefit to this engagment model. Note, if you drag and drop the content at the bottom of the stream, leads will receive this last. If you add it in to the top of the stream, the next time there's a cast sent, it will be that content because you are saying that is the top priority. Just remember from then on that would be the first thing a lead added to the stream would get.
-Kim