Actually, all you need to do is apply the Surface Replicator to the terrain - no need to convert it.

Within the Replicator is a Shading domain function (right side of panel), and just under that is the option to "Use Shader". When "use shader" is activated, the surface replicator automatically creates a new shader specifically for distribution - called the distribution shader oddly enough. With this new distribution shader, you can drag it onto your terrain temporarily and then paint on it, import shader maps etc. After that, you can put the terrain shader back on the object but now you have a new shader for distribution. If you choose not to paint on the distribution shader, you can instead use any of the procedural functions to set a black and white map for distribution, including all of the terrain functions.

Pretty tidy. 

Mark