Hi,
i think this is not possible. From security reason the WordPress Media Manager denied an Upload of webp images in 5.1.x. One Version earlier – it was possible to upload an webp.
So the solution to this great plugin – convert it on the fly and place it to a “private” directory. With an regex rewrite in .htaccess, it is possible to deliver on existing converted images from jpg to webp format.
It’s simple and amazing!
I love this Plugin.