I cannot see how that would work. The US spec Diesel has a specific exhaust and urea system that would not be compatible with the X6 35i system. The DEF and other components make this impossible.
This is also the reason you don't see any aftermarket systems for the D. However it is possible to bypass this system but there would be alot of coding issues, not meet CARB and EPA standards, nor be street legal in most if not all states. These systems are in place for most Diesel trucks in the US but are for, "off-road use only." It simply isn't worth it at this time. Plus the plumes of black smoke that would pour out of you X5 would make it look like a POS.