You can sure add a pot where you show it, but be advised that like burdij has said, you won't be able to drive a transformer off the wiper, the driving impedance will be far to high, and very load dependant as to the sound.
What's your reason to do this?
If it is to "get a sound" by driving the preamp harder, and then attenuating the output to fit your A-D input range, well, you're not going to get the results you want, because this preamp sounds pretty much the same right up until it clips, then it gets ugly instantly. That's one of the reasons the rail voltage is fairly high, you just don't get close to clipping that way.
If what you want is a continuously controllable gain (within reason), you'd be better off putting a 2K pot wired as a rheostat with a 750 ohm resistor in place of the feedback resistor, or maybe 2K and 1.5K to give you a little up and a little down from the switch gain setting.