Nope. This is a basic law of physics. They are
rigid and to think they can be bent is foolish.
So how far are you willing to take this? Negligible input with power output? You do
realize what you are saying here right? Now I can understand that there may
be more air flowing than necessary so a paddle wheel slowing things down wouldn't matter. If that's the case then it would be more efficient to change the fan or slow it down so there is less energy used by the fan which leaves more net energy to be used for something else which in your proposed generator would be produced but with a loss due to inefficiencies between the motor/fan and paddlewheel/generator.
There is no difference. Existing fan... Existing moving vehicle...