If you want to control a DC or stepper motor(probebly a better option) like a servo you can convert it into a one. All you need to do is mechanicly attach a pot to the motor, include that pot into an oscilator circuit. Then compare the oscilator output to the input pulse width(technicly not PWM more a changing a pulse with into an angular postion). Then you move the motor in the correct direction to remove the error between the pulse widths(i.e. the input(your signal) and the oscilator output(current motor postion). Like the earlier poster said though you could just buy a servo. Unless you want to use it for something big. But basicly thats what servos do. The actual electronics could be a bit of a project tho since it won't be some little DC motor. There would need to be all kinds of signal condtioning unless you were gonna use a seperate power supply(for control equipment), cos a big motor jittering about would reset any micros. So you'd probebly need to do it all hardwired. Unless you use 2 seprate supplys. Then again if you did use a stepper maybe that wouldn't be such a problem. Sorry I've probebly been no help. Go there
http://www.seattlerobotics.org/guide/servos.html but I think thats what you were meaning yes?