¿Es posible agregar una leyenda en la plantilla Story Map Cascade?

No, it is not possible to add a legend in the Story Map Cascade template as dynamic legends are currently not implemented. However, it is possible to add an image of the legend in the template. In ArcGIS Online, take a screenshot of the legend from the map viewer or create a graphic image of the legend manually. To add the image of the legend in the Story Map Cascade template, refer to ArcGIS Blog: Add images from your local drives to your Story Maps. The image of the legend is added as an in-line image in a sequential section, or in the immersive panel if the map is in an immersive section.

Note:
The Story Map Journal template allows adding a map and a legend to a section. For more information, refer to ArcGIS Blog: Configuring Story Map Journal Legend and Overview Map.