You missed that boat - Tamron was doing that eons ago with their Adaptall systems. It was easy to do when everything was mechanical - aperture setting tabs on the lenses telling the body what was going on, basically an all-or-nothing lever arrangement doing the stop-down, manual focus, etc. With everything being electronic and "you'll need to reverse-engineer that" proprietary, a simple swapable arse on an otherwise generic lens really isn't in the cards anymore.