You can add as many steps as you want to your Smart Vid, simply by adding choices to the question on each video:
In the canvas, hover over your video and click on "Edit step settings" (pen icon)
Click on Question to edit it
Add a video from your library by using the choices menu. Once a video from your library is added, a new video step will appear in your canvas
You can go back to the canvas and click on this new step to add another question and more choices
Note: The added video is a duplicate of the video from your library. The original video in the library stays completely independent and is not impacted by any changes inside the Smart Vid you're working on.