If you can live with changing the velocity, go to Select | Select by Filter. In the note range, leave the lowest number that is there by default on the left, and put C5 (Middle C) for the highest number on the right—or use B4 or whatever note you want for the highest note in the left hand. This will select all the left-hand notes.
Then you can scale velocity to make the left hand 80% or whatever percent you want.
Note that many piano sample libraries make the lower velocity notes less bright which is correct if you want the effect on playing softer with your left hand. If you don't want to lose the brightness of the higher velocities, then CJ's method is probably what you want.