Focal reducers (FR) are well known in astrophotography with telecopes. Here, the FR is basically a positive lens with a focal lenght typically around 200-300 mm, reducing the effective focal length of the telescope by some 0.5X to 0.7X.
See for example here, Focal Reducers:
Useful Formulae
The reason that this works with telescopes is that one usually has an ample amount of back-focus (a long draw tube) that allows the sensor to be brought closer to the telescope lens. This is not the case with a DSLR with its fixed registration distance.
IN THEORY one could do the trick by means of a couple of lens groups that would 1) reduce the size of the image formed by the camera lens while projecting the image further away from the lens mount and 2) reversing the image and thus projecting the image even further away.
See for example here, Eyepiece Projection:
Useful Formulae
But, as already said by rrstuff, that would most likely be impractical.