Total beginner here. 
Can I use a Nano to supply a low voltage to a dc motor?
I am building a wind turbine which I want to turn really slowly using a Micro Metal Gearmotors - Precision gearbox with N30 motor - RIC MOTOR.
My model railway has 12v and a 6v independent power circuits.
My question is, could I use a Nano to supply less than three volt supply to the motor?
If the motor runs with almost no load, you might be able to drive it directly from the Nano using a series 220 ohm resistor and a flyback diode on the motor for back EMF protection. That would make it current driven so it would have poor torque. If you're lucky that will not be too much current for the micro motor. Any smaller value resistor will overload the Arduino pins. Are you planning to connect to a digital pin to control it, or just power it on all the time?
No! Arduino pins are safe for about 20mA... the stall current of that motor is 300mA.
What makes you think an Arduino is in any way related to this? 
Serious question. Why do you refer to an Arduino (Nano)?
If you want a low voltage, you want a switchmode "buck" converter. Piles of them available on eBay and dirt cheap. Must I search them out for you?
6V supply, MOSFET driver for the motor, PWM from the Arduino at 50% duty cycle. End result should be pretty close to using straight 3V to the motor.