I had the same thought but the air suspension isn't partitioned off from the inflation system- which is why you can have problems if you leave the hose hooked up and an open valve.
Connecting up an air horn system to the air suspension would result in flattening the air bags anytime you honk the horn
Ok- not that extreme but it would cycle the compressor. Another option would be a reserve air tank charged by the air suspension compressor but dedicated to the horn system. That would require a solenoid valve and a little more electrical work to control the system and for pressure regulation.
In the end you're adding more complexity and using a very expensive compressor for a yeoman task, I'd stick to a separate compressor with it's own regulation.