What is sunglass lenses that attach to prescription glasses called?

Different options. There are a few different options for a sunglass tint added to your existing glasses. The most common are "clip-ons" which are usually magnetic attachments in the same shape as your frame you can put on outside. Also there are newer transitions lenses that are photochromic and will darken depending on the uv radiation from sunlight. See your optician for all the options.