Can you change or add a diffrent lens to a cannon powershot S5IS

NO, you can't change the lens on a Canon S5iS camera, but there are add on lenses that "KIND OF" do do the same thing by screwing over the existing lens via the lens cover threads. Most common are the wide angle attachments, because the telephoto capability is already pretty good.

All slightly lower quality and definitely increase the light requirement.

Yes you can. But you have to visit a canon service center and order them to your lense.