No. The Leaf won't start unless it detects a key inside the car, and if you manage to hack that it would be the same as just leaving your car keys hidden in the car all the time. You could do that if you really, really wanted to, and lock/unlock the car with your phone, but then if anyone broke in they could just drive away.
Also, I would not want to trust the Nissan app to be my only way to unlock the car.