[Ginto8]

Using whileHeld() would be an equivalent approach than making whenPressed() turn it on and whenReleased() turn it off. To make it toggle you need a command to somehow change the shooter's state based on a current state. I expect your shooter has some setRunning() and isRunning() (the methods might be different -- the important part is being able to tell whether or not the shooter is running). The logic you want (probably in initialize()) is this:

if(shooter is shooting) {
    turn shooter off;
} else {
    turn shooter on;
}

If your shooter is a PIDSubsystem, you can do something like this:

if(shooter.getSetpoint() != 0) {
    shooter.setSetpoint(shooterSpeed);
} else {
    shooter.setSetpoint(0);
}