Painting Biomes - Minecraft does the rest. Is that possible?

So here is what I am trying to get done:

I want to create a map, which I paint in a new world where the Biomes should be. Then I want Minecraft to create the terrain accordingly. I.E.: if I painted "Extreme Hills" I want Minecraft to create extreme hills there. If I paint rivers, Minecraft should create rivers at that spot.
I gave that idea a try via the "populate" option. But all it does is placing trees and grass etc., not changing the terrain. But the Terrain is what I am interested in, as I don't want to create all the landmass, but only in special areas.

So here is my question: is this possible in WorldPainter or do I need a script? If I need a script... is there one for this purpose?

Thanks in advance, Iden