I found these pix and instructions a couple years ago - thought they might be handy to have "just in case" . . . . .You can remove the passenger side panel from the shift console and push the latch with a screwdriver. That releases the shifter from Park and allows you to shift to Neutral.
Volt Shift Interlock Override
When working with a Volt with a discharged or disconnected 12-volt battery (the vehicle will not start), the shifter cannot be placed into neutral without first bypassing the shift interlock. Here’s how:
1. Remove the right trim panel from the lower portion of the center console (see figure 1 below). The panel is held on with spring clips. Grip from the bottom, and pull out to release the lower portion of the panel, then carefully work your way around.
2. With the panel removed, locate the release lever just below the center stack, above and slightly behind two wiring harness connectors (see circled area in Figure 2 below).
3. Place your figure in front of the lever and push to your left, toward the rear of the vehicle (see arrow in Figure 3 below). The lever will move about a quarter of an inch. Hold the lever in this position.
4. Press the button on the front of the shift lever and move the shift lever to the neutral position. Vehicle will now roll freely, and can safely be loaded on to a flatbed platform.