It entirely depends on how the existing switch is wired. You need a constant hot and a neutral wire. It would be helpful if you can check what wires are in each location - what colors, and how many actual cables are coming in. You'll need to turn off the power at the breaker panel and physically remove the switches (don't disconnect them, just pull them out), as well as the light fixture. Compare to the diagrams below (or at the original source), or ask here again and we'll try and help you out (labelled pictures == very helpful).
There are several ways to wire 3-way switches, but what you'd be concerned with is the type where power comes to the switch first:
In the above diagram ,you can add another switch to the one on the left - you'd connect to the white and black wires coming from the "power source" line.
On the switch on the right, you have a neutral, but you don't have constant hot -- hot is switched, on either the red or black, depending on how the first switch is flipped.
Another common way the switches are wired is the power goes to the fixture first.
In this case, there is no neutral at the switches. (Also note, the black tape on the white wires indicates this. White is, by code, always neutral, and has to be marked if it's used for a switched circuit. That said -- keep in mind that not everyone follows code.)
If you're missing the hot/neutral, the only option you have is to run a totally new wire, or possibly (depending on how it's wired), you can convert the 3-way to a single switch, and then re-purpose the wires to supply constant hot and neutral to the new outside switch (effectively, one of your 3 way switches would get converted to control the outside light instead). This still may not be possible, and definitely isn't a beginner task - you need to understand electrical fairly well, and map out everything involved with this circuit.
Since your light is on a different circuit than the outlet, you'll need to run a switch loop from the light to the 2-gang box using 14/3 (yes 3) cable, with black as the hot, red as the switched hot, white as a spare neutral, and the bare or green wire as well, the ground (aka EGC) :)
In the light box, you'll take the black that currently goes to the fixture hot and connect it to the black of the switch loop instead; the fixture hot then gets wired to the red wire of the switch loop. The switch loop white and green wires get wired into the existing white and green wires coming into the box.
After transposing the existing GFCI from the old 1 gang box into the new 2 gang box, you'll want to leave it alone from here on out. To wire up the switch, you connect the brass screws to the black and red wires, the green screw to the EGC, and simply wirenut off (i.e. put a wirenut on the exposed end of) the neutral on the switch loop -- it's there for future use by say a motion sensor or lit switch as per 404.2(C) (neutrals are called 'grounded conductors' in the NEC, btw, if you're a Code newbie and scratching your head at this :):
C) Switches Controlling Lighting Loads. The grounded circuit conductor for
the controlled lighting circuit shall be provided at the location where
switches control lighting loads that are supplied by a grounded general-
purpose branch circuit for other than the following:
(4) Where a switch does not serve a habitable room or bathroom
Best Answer
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.