Either one: search for MarioG's tutorial on mixing terrains using aplha maps. or two: duplicate your cliff/mound, erase everything on the duplicate except the whitest parts in Terrain Editor, raise a bit on Y axis what is left, and then give it a different texture. Or use Spikes in Terrain Editor and give it a different texture. or three: if the area you need to cover in grass is small, just download Grass from grass3d.com and create meshes to put on the top of your mound.

-- erlik