I have been trying to figure this out for a while. I have already created a terrain with a base material from the starter pack. I want to select say “Moss” and paint portions of the terrain using the already created materials in the materials folder of the starter content. Do i really have to create a new material and texture it and make each instance node in order to just paint on my terrain?? Surely there has to be an easier way to do it. I just want to use the already created materials to paint on my terrain with, im not wanting to fill the terrain with just one material, i already know how to do that, a step by step direction would be nice.
The Docs have a step by step for everything
No way around making one.
You can CopyPaste the material that you like and plug it to the Layer Blend of your LandscapeMaterial 
That’s as easy as it gets.