I realized that I might need to displace a few surfaces in the objects I wish to make. However, my first trials indicate that Terragen fakes displacement and instead of displacing the object per se, it simply applies "fake displacement". The surface does look as if it was rough / indented but in fact it's just a flat surface with some depth effects applied...
My question would be, how to make real displacements on a surface in a way that it actually changes an object's shape. Or should that be done while actually building an object in, say, Blender not in Terragen?