+1^ for rolli59 & ct1615
I have a CM Elite 335 and both the intake and exhaust are threaded for 80mm. 90mm and 120mm like rolli said.
You can definitely remove the duct and place and 80mm by the CPU like ct1615 said. Although there are no holes meant specifically for screws by the GPU vent you probably could stick another fan there and thread it through the exhaust holes.