Alpha blending with material functions

I have been following the tutorial by Ratsgame to create a decent landscape material using material functions. I like the result. I am thinking it would be cool to be able to blend material functions through an alpha blend. I would like to mix maybe grass and rocks in a controllable, scalable way independent of the material function nodes. That way when you look out on a field of grass there would be patches of rocks interspersed through the alpha mask.

Any way to do this? Apparently within the material using material functions, things like lerp are not allowed.