Have you already set up Track Quick Controls? (If not, see pg. 653 of the Cubase 8 pdf for how to do so).
Once you have one of your external MIDI faders (let’s say CC#3) connected to one of the eight available slots of Track Quick Controls in Device Setup>Remote Devices>Track Quick Controls, go back to the project window, select the desired MIDI Track, open the Quick Controls pane of that track’s Inspector, click on the corresponding slot, and navigate to “MIDI Modifiers>Velocity Shift” (and don’t forget to actually switch on Quick Controls 
). Now your incoming CC#3 will adjust “Velocity Shift”. You can Write the automation of this fader.
Sorry it isn’t simpler .