In Farseer 3, if the body is connected with a revolutejoint you can use the motor to track the angle. It is explained in the
Otherwise you would just gradually increase the angle by a certain amount until it reaches the required angle. If you want it to look smooth you can use a Cos function.
In Farseer 2 there is an AngleJoint and a FixedAngleJoint which keep an object at a certain angle, which might be usefull to you.
To get the short way around I think you can use MathHelper.WrapAngle, which wraps an angle between -Pi and Pi.