You sure can. Many ways to do it - one is to select all of the notes by right clicking in an empty space in the PRV and lassooing them, then entering the new velocity value in the event inspector module on the control bar . You could also just select the entire clip in track view and change the velocities for all notes in the clip the same way.
Bruce.
Sonar Platinum 2017-09, Studio One 3.5.3, Win 10 x64, Quad core i7, RME Fireface, Behringer X32 Producer, Behringer X32 Rack, Presonus Faderport, Lemure Software Controller (Android), Enttec DMXIS VST lighting controller, Xtempo POK.