What is the source data for your terrain provider? If you’re using heightmaps you could edit that source and re-generate the terrain.
This forum thread implies using a shader to alter the Z coordinate of the tiles may work but I haven’t seen an implementation of this: How to flat the overlap area between a polygon and a 3D tiles to a specific height?.
I’m curious to hear more about your goals with this feature. Is the idea to flatten it so you can add new data there instead?