This is very simple, because Path Painter does not alter the (texture) layers or shaders of the terrain in any manner. It just works with whatever was set for the terrain.
For this reason you don't need to do anything out of the ordinary to edit the Terrain Layers. Edit them as you would normally do in Unity via the Terrain Inspector (under Paint Texture)
See the Unity documentation for Terrain Layers for more info: https://docs.unity3d.com/Manual/class-TerrainLayer.html