Wide-angle- or even fisheye-converters exist, popo! But not for DSLRs. They of the variety that is mounted in front of the lens - not between thelens and the body. You can buy those for vidoe cams and p&s cameras.
Whether it is physically infeasible to build one that mounts like a tele-converter? I don't know, but I think you can build a contraption that demagnifies the imge from the primary lens. This will naturally increase the brightness of the image. Unfortunately you will demagnify the image-circle of the primary lens too. So don't expect this to wrok well with DX-lenses. But with FF/FX-lenses no a DX/APS-C body you could easily shrink the focal length by 1.5x without getting in to problems with the image circle. Thus your 50/1.4 could be converted to a 33mm f/1.0
Btw.: Modern wide-angle designs (of the "inverted tele-photo" type) already demonstrate that you can have a 10mm focal length although the back of the lens is 30mm away from the sensor.
Thomas (beware: Nikon-fanboy and moderator!) My Lens Reviews
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