@Pronoob123: Typically, if an option is greyed out in the settings of an app, it is an embedded feature, and cannot be customised. While it isn't quite the same, I recommend heading to Settings > Advanced features > Motions and gestures > Double tap to turn on screen > On > Double tap to turn off screen > On. This will allow you to wake the phone when it is locked/idle by tapping the screen, and should hopefully serve as an extension to the Back-Tap feature.
