I was told that adapters exist that allow you to mount an additional lens on a non-threaded camcorder. Are these adapters worth it? How common are they?
I altered your question pretty significantly. Product recommendations and shopping for you are explicitly off-topic for this site, however there was other important related to the products you are looking for that I thought were worth communicating enough to edit the question a bit so it could still be touched on without directly closing the question.– AJ Henderson ♦Jul 1, 2015 at 15:07
Such devices do exist a reasonable amount of the time, but they rarely produce much in the way of quality. There are exceptions, but they are fairly rare. They do have their uses, but be aware that the results are most often marginal. Your best bet to find them is generally looking around the various smaller NY/NJ photo/video shops that carry a variety of knock off products from around the world.
Make sure you are aware of what you are getting yourself in to with them.