If I'm understanding you right, you're going to need some kind of feedback from the motor to tell when it's at 180 degrees.
Servos have built in Potentiometers and circuits that allow for feedback, which allows for the precise control of 180 degrees. You could also use an Encoder as shown in the link below, but generally cost more and require more work on the software side.
I typed "dc motor to servo" in Google, 541,000 links. Number one:
http://www.truetex.com/servomod.htm