If you're doing it yourself, you can only add a potentiometer to the throttle to limit the speed. The PAS function is on or off, so nothing to adjust.
There is one more thing you can do to your bike to legalise it. You can add an external microprocessor to control it. There are two commercially available ones. There's the Cycle Analyst from Grin, which is very expensive and the Speedict Mercury, which is about half of the price of the CA. Contact Speedict about the Mercury. I'm sure they still sell it, but not easy to find on their website They both accept a PAS and throttle signal that they mix into a single programmable throttle signal.
Any college student that knows coding should be able to knock something up for you for a few dollars using an arduino or PIC chip. You program three inputs. One is analogue 5v for the throttle, two is analogue 5v for the pot, and three is for pulsing 5v for the PAS. The logic is simple.
Set a maximum allowed for the output signal, which will be around 3.5V to 3.8V to give 15.5 mph max.
If input 1 is detected, send it directly to the output only if input 3 is pulsing
If 3 is pulsing, output = the input from the pot up to maximum allowed
The whole thing can be powered by the 5v on the throttle cable. Additionally, you ned a capacitor accross the 5v supply to take off any noise, a pull-up resistor on the PAS signal wire, and most microprocessors can't do analogue output but can do an equivalent PWM output for which you need a resistor and capacitor as a low-pass filter to smooth it into analogue. Any electronics student will know about these things. The arduino forums should be able to help you out with an exact parts list and coding if you ask them nicely.