Tracks Creating GRASS FX for my track

[Larsess]

Hello, I'm creating a track and I was wondering if is there anyway to add Grass in certain textures? Because most of the map is sand and I need to add grass in determinated parts of the map. I need just add it to the texture called "ground3". I'm using Race Track Builder

 

[Larsess]

As I'm using RTB, backround it's just 1 Material (1GRASS0) and I don't know if I can divide the mesh.