It is currently to render a polygon which is not planar? I only see an option to increase height above ellipsoid, but no option to respect origin z values for each corner.
Here’s an example of a non-planar polygon: http://cesiumjs.org/Cesium/Apps/Sandcastle/index.html?src=Polygon.html&label=All
Polygon positions are Cartesian3 s. If you need to make more complex 3D geometry, consider making a custom geometry type: https://github.com/AnalyticalGraphicsInc/cesium/wiki/Geometry-and-Appearances
Hope that helps,