Generating a Displacement Map in Unity3d?

So far I have been able to spawn randomly sized spheres on the go. but I want these to look like asteroids, and not perfectly smooth spheres. I would like to distort the geometry of an object as it is generating in order to make it feel more like you are actually flying among asteroids.

I suggest you look at the procedural examples project in the asset store - particularly the heightmap generator and crumple modifier to show how to deform a mesh in code at runtime.