You can add a 100k Ohm balance pot in front of, before, the volume pot.
Be advised, wiring of a true balance control is not as I first thought. Carefully draw a wiring diagram after you measure how it works with your meter.
The last one I installed took 3 wiring diagrams before I got it right.
Here is the kink, when you fade to the left channel the left channel is not changed, the right channel lowers. When you fade to the right channel the right channel is not changed the left channel lowers.
Good luck !
