Tesla considered making this a retrofit option in the past and decided against it. There are too many expensive parts that need to be changed. Unfortunately, if you want to go from coils to air you're going to need to buy a new Model S.
You could get it bagged after the fact, but it wouldn't be controllable through the touch screen. As for cost, it would be hefty probably about 6k all things said and done...not sure its worth it as you'd have basically no warranty.
If you're concerned with ride height consider a coilover option, if you're considered with stiffness, get a different spring.