Can I use a stepper motor driver to control a brushed motor?
The motors/drivers you've specified are an excellent match, as long as you realize that you'll need to provide a fairly high-speed PWM (Pulse Width Modulation - say, 10 kHz) signal for your speed control. The drivers are rated for 1.2 amps average, and the motors are claimed to run at ~100 mA, although I suspect that's with no load and they may well draw more when trying to do useful work. Plus, you won't get full speed under load.

What you cannot do is simply hook up a pot or variable voltage control input and get variable speed. You can work by varying the motor drive voltage, but this is a bit more involved, since it requires a relatively high-current amplifier or power supply, and is clearly not something you've done. Like I say, you'll need to learn about PWM for speed control.