Support for polygons

I've seen a couple of posts about this, but wanted to ask if anyone had had any success through any method adding polygons to the basemap of a storymap. I my org wants to tell a story about environmental degradation and deforestation in South East Asia, and we need to be able to have a map that shows concession areas for logging and national parks. Is there anyway of doing this? I can't see how change the basemap in the JavaScript... http://storymap.knightlab.com/advanced.html