As far as I know, if you were to 'mod' a motor to control its speed, you'd basically be making a speed controller, which work by (as far as I know) sending pulses of current to the motor with varying widths (Pulse Width Modulator) in order to change the 'average' DC voltage. I guess you could use some kind of transistor/amplifier circuit if you just wanted to control the voltage over the motor (which is proportional to the force given with a constant magnetic field), but I have no idea how well that would work. I'm new to roboteering too, but I do study this sort of thing at Uni.

So TL;DR: no? I don't think so, at least not conveniently.