Hi Wouter,
You can already add YouTube (or Vimeo) videos to your Cascade stories; just paste the URL of the video in the Link to Content tab.
Also, here's a blog post about all the features of the Cascade Media Picker:
Using the Story Map Cascade Builder: Media Picker | ArcGIS Blog
Owen
Owen,
Thanks for your post. I was using the wrong URL, after reading the blog post everything went well,
Thanks!
Wouter