I researched this quite a bit, and tried quite a few lenses ( including the abovve mentioned Olympus WCON-08 ) and the only one I found that would provide satisfactory results was the Sony VCL-HG0758.
Be aware that you can't just screw it on to the end of the prime lens of your projector, you are going to have to build a custom mount for it. The mount must allow you to adjust the both the height and pitch of the lens. Positioning the lens in front of the prime lens of your projector is a finikey process involving the distance between the prime lens and rear-element of the add-on lens, the height above ( or below ) the prime lens, and the pitch of the add-on lens. Unless it's exactly right, you'll see either geometric or focus distortion on the projected image. Also beware that this lens is heavy. The mount has to be solid, as well as the mount's connection to your ceiling ( or table, which is how I used it ). You can find the Sony lens online for about $250.
The expensive Navitars are easier to setup as they are designed especially for this purpose with a very large rear element. There are also mounts available for them. But, this of course comes at a cost, in this case over $1K with the mount.
If your throw is constrained because of the depth of your room, a MUCH easier solution is the 'mirror trick'. Simply point your projector at your back wall ( about 2 feet away ), and hang a mirror where the beam lands on the back wall. This will increase your throw distance to the length of your room +2 feet. Using a first surface mirror from a junked RPTV is ideal, but I've had good results with just a regular good quality mirror. You just have to set your projector to 'rear projection' mode so the image is flipped horizontally to account for the mirror.
Hope this helps,