It's possible.
I sincerely doubt Maxwell will be able to help you. It's also probably no practical to attempt to change the gears in this to change the speed.
What is possible is to do what Zeehag suggests, which can be done with a DC motor speed controller.
What Zeehag is doing is pulsing the switch on and off to get the average speed he wants. A typical DC motor speed controller is a pulse-width modulator (PWM) that pulses the motor on and off very quickly, varying the width of the "on" pulse to get the speed desired.
It may be expensive, since windlass motors draw a lot of current, up to 100A.
I saw one "hobby" PWM controller on eBay for $68, though. Search for "12-40V 100A 1200W DC Motor Speed Control PWM Controller" on eBay. Would need an enclosure, and then you'd dial the potentiometer to the speed you want and just hold your windlass switch down for that speed. You'd have to go below to adjust the speed (assuming the enclosure is belowdecks in a dry place).
CAVEAT: I haven't done this, and don't know anyone who has; if you don't know what you're doing with electricity a and electronics, especially around high current circuits, you should not attempt this alone. Also, no telling how the Maxwell motor will respond to this, in terms of operation, thermal characteristics, and life span.