Proportional control of 0-12 V DC power source with 0-5 V DC analog signal
You can use a power transistor, but not on it's own. 

You will need some form of control circuit to properly apply power to the motor in proportion to the ADC voltage. 

Also, the linear method you are thinking about, basically a voltage regulator, is horribly inefficient at lower speeds. A pulse width modulated control system that turns the power full on and full off at different proportions at a high frequency, is a much better solution.

However, a very simple, one of, solution can be fashioned this way...







simulate this circuit &ndash; Schematic created using CircuitLab

The circuit actually adjusts the maximum current than will flow through the motor, and consequently the maximum torque available, which in turn limits how fast it can go. The POT needs to be sized based on the worst case \$\beta\$ of the selected transistor, and you would adjust it so that at max ADC value, you get maximum current through the MOTOR and the transistor is close to the saturation point. R1 is only there to protect your ADC in the event that you crank the POT all the way to the zero resistance end.

Speed control will NOT be linear with ADC value though since it will be dependent on the mechanical load, which for a fan, is not linear with speed. Also, to get the fan started, you would need to begin with a high torque setting then back off some time later.