Hi westergard,
Currently in Terragen you can not apply a shape shader to a light or create a population of lights. You could however, create a light source from geometry in the shape of a ring, apply a Default shader, and set its luminosity value so that it emits light. The rendered results would look best with the Ray Trace renderer as opposed to the Standard renderer, especially if it needs to illuminate other objects in the scene.
There may be other methods to achieve the look you're after; do you have any reference images for what you're trying to accomplish?
