Camera Roll allows an image to be selected for compositing with the current images in the Panorama view.
Camera Roll is rather slow coming up, especially if you have thousands of photos. Wait for it to show up. Select the image you want to be composited. Once the image appears in its own view, swipe the image(in any direction), the image will be placed in the Panorama Workspace to be composited with others. You can also rotate the image and zoom it in/out to scale it, before moving it to the Panorama Workspace.