I want to extend the default HeightField viewport shader, after digging in the Houdini files, I find the default HeightField shader is defined in
C:\Program Files\Side Effects Software\Houdini 17.5.397\houdini\glsl\material\GL32\beauty_game_lit.frag

whatever shader I assign to the HeightField, it will use the beauty_game_lit/unlit anyway. How can I overwrite the default viewport shader for HeightField? I just want to blend more than 4 texture for HeightField just for preview.
Thanks.