How does one create a spherical light source? I would like to create a sphere which radiates light.

I think there's a basic misunderstanding about how Stager works, and 3D in general. When you import a background, it's just a flat background. In itself, it has no depth and does not interact directly with anything in the scene, including lights. If you create a Point or Direction light in a scene, it can't "hover above the landscape." It can only hover above the ground plane.

A reasonable analogy, not entirely accurate, is a painted backdrop on a big stage. You can light the backdrop scene, but you obviously can't hover any kind of light in the scene. It's a flat image, just like the background in a Stager scene. When you use Match Image inside Stager (or Dimension), the Sensei AI analyses this backdrop and adjusts the ground plane and perspective to match it as far as is possible. Notice that it's the ground plane and camera that are being adjusted. Even though it may appear to be on the background, a shadow cast by an object will fall only on the ground plane or other 3D objects in the scene. If there is texture or discontinuity in the background image, the shadow cast by a light in the Stager scene will not match that. The background also will not change to match the color of any other lighting in the scene.

This is one of the less-intuitive aspects, for a designer, of working in Dimension or Stager. When you change the camera angle to, say, an overhead view, the background doesn't change. Stager tries to avoid this by offering a "viewport camera" that has no image background, so you can build your scene without being fooled by fake scenery.

Here is a screen snippet to illustrate the point. It shows a model lit by a pale green directional light. The model material itself is white. All the environment lighting, including sunlight, is turned off. Notice that the background has not changed to match the color of the light, and the shadow of the model does not match the background at all: it is being cast onto a flat plane. The small circular icon near the top left is a directional light from the "Physical Lights" section of Starter Assets, which is the only illumination on the scene.

Did you try this feature?