Is it possible to use add-a-wire with my current configuration to add a C-Wire?
According to the ADD-A-WIRE™ Owners Manual, you should be able to use the product to achieve your goal.  I've never used this product, and do not endorse it in any way. I have no idea if it actually works, I'm simply going on the claims made by the manufacturer.
This is the diagram from the manufacturer.



This is how I'd wire it up.


R and W stay the same, at both the furnace and the thermostat.
Switch the BLUE wire to C, at both the furnace and thermostat.
Connect the ADD-A-WIRE™ device in the furnace as described in the installation instructions, but connect the BLUE wire from the ADD-A-WIRE™ device to the GREEN wire in your cable.
Connect the BLUE wire from the ADD-A-WIRE™ Diode 'Y', to the GREEN wire from your cable at the thermostat.
Finally, connect the GREEN wire from the ADD-A-WIRE™ Diode 'Y' to the G terminal of the thermostat, and the YELLOW wire from the ADD-A-WIRE™ Diode 'Y' to the Y terminal of the thermostat.

WARNING: Use of this product is at your own risk. I have never used this product, and will take no responsibility for any damage it may cause. Be sure to read all manufacture documentation before using this device.