I would follow mister-e's suggestion and use a divider (2 x 10k resistors) to halve the voltage to give you the 5 volts and then feed this into the PIC.

You could use the POT command to give you a value which is then used as the delay for the pulse train on the output, something like (I've not tried this but it should give you the idea)

Code:
scale = 10
Pot GPIO*,scale,D
pwm GPIO.**,D,1