Adding Landsape materials

I am having a problem figuring out how to add the terrain materials that I’ve bought on the marketplace for terrains to the existing material set up that is already in place on my map . I set it up making the existing material, adding landscape layer blend and adding each type rock grass placing textures and normal’s and painted my map so I’m not to sure how to add a completed material on to that because I only see one slot for a material when selecting the terrain