Well, I havent used terragen in a loooong time, and even then it was a quicky to create heightmaps.
Since there are no textures in the file, only parameters like altitude,colors, etc. I can only think of two options.
1. Terragen exports the texture "maps". ie like the heightmap.
2. Posiotion camera to view the texture and render out.
Sorry thats about what I can think of. If you explain a bit more, maybe I can think of some other options. For example, "I want to a terrain texture to map on my terrains" or " There's still small part of the terrain I want to use and tile for my matrix"...