In the Extreme NXT book, we show you how to make several custom output devices using the PCF8574. Below is a simple interface for a stepper motor. The ULN2003A provides enough power for a small stepper. You will need to supply a separate DC supply to power the motor. A NXT-G program using the I2C Write custom block produces the pattern needed to rotate the motor clockwise is shown below along with a short You-Tube video demonstrating the interface.
Stepper Motor Interface