First off, although the sellers label it as a wide angle lens, in reality what you are considering purchasing is more properly called a wide angle conversion lens because it screws onto and converts your existing lens to cover a wider field-of-view. In general the products sold in this price range aren't very good, and that is putting it nicely. There are some more expensive wide angle conversion lenses, such as those made by Raynox, but the largest size I'm aware they make fits on 58mm threads. And even those involve a fairly significant compromise in image quality.
The step up adapter should work on your 50mm lens, but I would be very surprised if the image quality were even anywhere near as good as your 17-85mm zoom zoomed to around 35mm. The magnification provided by the conversion lens is listed as 0.45x which should produce a field of view of around a 33-35mm lens when combined with a 50mm lens. Remember magnification is expressed in terms of coverage area, not focal length. In return for that modest improvement in angle of view, you will almost certainly see a substantial increase in vignetting, distortion (the fish-eye effect), and chromatic aberration (color separation that makes objects near the edge of the picture look blurry). Here is a well written review of a similar lens that goes into far more detail and provides test shots comparing using a wide angle conversion lens with a wide angle lens.
You would probably be much happier with the images you get by saving up for something like the EF 35mm f/2. Although it has been replaced by a newer (and more expensive) IS version, it is still available from many online retailers (at least here in the U.S.). Even the very inexpensive Yongnuo 35mm f/2 for Canon EF mount should outperform an EF 50mm f/1.8 + wide angle conversion lens.
