Is there anything wider than the Sigma 12-24mm? Are there any attachments I can put on my camera to get an even wider focal length?
The widest rectilinear lens available for any full-frame is the Canon EF 11-24mm F/4L USM. To get a wider field-of-view, you need a fisheye or stitch multiple images. Like many extremely wide lenses, a 11-24mm lens does not allow attachments at the front, only filters at the back but those will not help you get a wider angle-of-view.
Rectilinear and wider that 12mm on full frame is hard to get, but also hard to use. It's not usual to need more than this exceptionally wide angle in this context. The biggest issue with wider lenses is that they are more likely to suffer from flare ( just trying to avoid the Sun or a bright light source in shot can be impossible ) and are ( as a general rule ) more prone to distortions.
If you need a wider image then consider stitching panoramas in software. There's very good free software, like Hugin, that will do this and you normally just need overlapping images to do it. Stitching means you can employ images shot with a lens less prone to distortion and combine them to make a final image that is more detailed and less affected by optical distortion.