1. A concise explanation of the problem you're experiencing.
2. A minimal code example. If you've found a bug, this helps us reproduce and repair it.
3. Context. Why do you need to do this? We might know a better way to accomplish your goal.
4. The Cesium version you're using, your operating system and browser.
Do you mean that you have 2D data (polygons etc.) and you want to display it in 3D? If so, you should be able to just do that with Cesium’s entity API. If you mean something like extrude 2D shapes so they have thickness in 3D, here’s an example of that:
If that doesn’t answer your question, a bit more context about what you’re trying to do would help! A picture or an example helps too.