What's the best way to add a neutral wire for smart switch?
If you're going to run a neutral after the fact, you'll pretty much have to trace out the circuit, and run a new wire / cable.  This could be a real project.  

As mentioned in another answer and the comments, you could put the wireless switch in the circuit somewhere ahead of the lights where the neutral is present, and just splice the wires from the driveway switch.  There are battery-powered switches such as the Lutron Pico that you can install in a cover plate - you could mount that in the driveway switchbox, and things would operate as they always have, along with other wireless control.  

Of course Lutron makes some Caseta wireless switches that do not require a neutral.  They "bootleg" their neutral return on the ground wire, which is generally verboten, but UL allows it for the tiny currents these devices draw.