I think it'd work but it'd seriously compromise ground clearance. The forks are very different. The Duke forks are roughly as long but don't have nearly the same travel.
I was thinking about this mod at one point for my 690sm too. The problem is that as stated previously our bikes have more travel yet the same overall height as the Duke. Usually as I'm sure you kow when you add travel you gain height. With less travel and the same height the Duke will have more ground clearance, my guess is that it needs it for the under engine exhaust. What may happen is the 2.75" difference in suspension travel would allow the Duke exhaust to hit the pavement if you use more then 5.5" of the available 8.27" of travel. Not just off road but on any surface. I never confirmed this as fact but in looking at all of the dimensions it appears that would be the case. You may be able to get more info from KTM as to why the Duke has less travel but the same over height as the SM. Good luck, hopefully someone knows for sure if the Duke exhaust would work.you mean if you want offroad on the supermoto.. on road shouldnt make much of a difference since they are about the same height..