It's just the polarity that sets direction. Most expression pedals work the same way, Yamaha or not. You can usually set direction, ( polarity), span, resistance, either on the keyboard or the pedal or both. Polarity can be set on the keyboard, because sometimes you need it the other way for example using it for pitch bend where you want full pitch to be full up as opposed to volume where convention is full volume, full down.
So of course you can easily set volume in what I would call the reverse direction, just seems like an odd thing to do, completely counter intuitive and I have never heard of anyone doing volume that 'wrong' way round with a guitar or keyboard. So you can do it your way of course, but I honestly think you will soon work out it works better the other way because your brain will have problems relating less volume to more push and you may also have problems setting the pedal with your foot on the fly to get just enough volume, because with the polarity reversed it will go to zero volume if you press down too far.
Mike