If you mean you want to connect an older "standard" lego motor, you need to get one of those adapters. I think you should also be able to connect a normal motor to a motor adapter's outputs. You can't connect a motor directly to the NXT however, because the motor outputs also use I2C like the sensors. There has to be something to read the I2C and a H-bridge to power the motor.
Adapter link:
http://shop.lego.com/ByTheme/Product.aspx?p=8528&cn=17