Haven't looked at one recently, but it probably could be done with a dremel, cut the lense out, then tint it, then use some sort of adheasive to bond it back. Might be easier just to buy the tinted ones though.
when i was trying to do mine it seemed as it was one piece or glued together really well. i was planning on just sawing off the lens to clean it and make it clear and then glue it back on. but i never got around to it and i dont have that car anymore.