How can I add a light band to my arched niche in the wall?

You can't...But you can fake it. Use the door opening tool to create a square space keeping in mind the width of the wall. Then use custom modeling over the markings that show the square in the wall. Extrude to the width of your wall. Then create your arch niche and extrude that to at least half the width of the wall. Then (in custom modeling) assign a light band to your arch as desired. Just remember that the wall and the custom model need to have a solid material applied to it. Matching patterns on HS walls and custom modeling is almost impossible.