Yes, Pulse Width Modulation is possible with just the Expansion Dock, however, it is software-based PWM and there are no guarantees on accuracy.
See our article on using the fast-gpio program to control the GPIOs for details.
Software PWM vs Hardware PWM
Software PWM is implemented by a program that usually waits for a defined amount of time before toggling the GPIO output. This has the potential to be inaccurate since the CPU might be interrupted with other processes and tasks. Software PWM is generally good enough for dimming an LED but not for something requiring more accuracy, such as driving a servo.
Hardware PWM is done by programming a micro-controller that will output cycle-accurate PWM signals. The Servo Expansion has a micro-controller that can be programmed to provide cycle-accurate PWM.
PWM on the Expansion Dock
If you require PWM using the expansion dock and don't need it to be 100% accurate, we've built a tool that can provide software-based PWM.
fast-gpio pwm <gpio> <freq in Hz> <duty (out of 100)>
fast-gpio pwm 14 200 25
The command above will set GPIO 14 to do PWM at 200 Hz with a 25% duty cycle.
This means that during the 5ms period (P = 1/f), 25% of the time the signal will be HIGH and LOW the remaining 75% of the time.
This image provides a good illustration: