No it is not possible.
All of the switches are connected to digital-only pins, and you can't read an analog signal from them.
If you have good soldering skills, you could potentially solder directly to the MCU to make it work, but it is pretty tricky work, especially working with analog signals. Pin PA0 would be a likely candidate, but you'd need to know what you are doing to make it work.
Alternatively an a/d + shift-register could probably do it on a digital pin, but it would take a lot of coding to make happen. Easier to just attacxh it to the PWM in that case.
