How to add Hidden/TerrainEngine/...shader to ShaderVaraintsColloection

I need add some built-in shaders to ShaderVariantsCollection, like Hidden/TerrainEngine/Splatmap/Diffuse-AddPass.
But when I create a new shaderVariantCollection and click "Add shader" button to try to add this shader, I cant find any shader under the Hidden/TerrainEngine/.

How can i add those shaders to shaderVariantCollection?
Please someone tell me