Hi Scott,
Currently you would need to export the Terragen Sky project to Terragen 4, then add a new camera, or modify an existing one, with the fisheye lens and 180 degree field of view to get the results you're looking for.
Once you set up the new camera and renderer, you could save the nodes as a clip file. This would speed up the process for subsequent scenes following this workflow as you would simply load the exported file then import the clip file containing the camera setup.