Garbage Disposal Rewiring and Switch to outlet transition
How easy it is depends on how the feed and switch is layout in the switch box.  IF all three wires from the feed, (black/wire/ground) comes into the switch box, this is an easy setup.  In that box, the switch is removed and an outlet is wired in.  The feed is then continue to a new outlet under the sink.  Both outlets are hot, the old switch and the new outlet under the sink.  The air switch is now plug into the under sink outlet, a new hole drilled into the counter top or use existing hole for the air switch.  You would also have to replace the hard wire out of the GB with a power plug.
Under some local code, the GB would have to on a dedicated circuit, so this would be in violation of that code.  In my house before I remodel the kitchen, my GB was on a circuit with other outlets.