They do make a bracket to mount a single KU ( DirecTV ) LNB on a C-band type dish:
Ku2C3 Ku LNB Bracket for C Band Dish with 23 & 40mm Mount
* KU-band LNB bracket for C-band dish
* Allows mounting of a Ku LNBF on a C-band dish
* Has clamping for 40mm and 23mm LNBF throat sizes, fits just about any Ku LNB on the market
* Comes with 3 bolts to mount to your C Band dish
Dish LNB Brackets
I've also seen a bracket to mount a Dish 500 LNB on a C-band dish where the bracket tilted to get the correct angle, I have not seen one for the DirecTV KU/KA LNB's yet.