You can use a RevoluteJoint between the bones with limits enabled. To rotate the joint set both lower and upper limit to the relative angle desired. The solver will do the rest.
You can also use a motor on the RevoluteJoint and in each step calculate the motor torque necessary to achive the angle. Allows for finer control but you need some maths.
You can also apply torque/angular impulse manually with or without using revolute joint limits.
One problem with RevoluteJoints is that they are quite soft in my experience. Latest Box2D has a MotorJoint which looks like it is perfect for ragdolls. It should not be hard to port it to Farseer but you must use the latest
Farseer svn due to changes of solver parameters being passed around (or wait for the upcoming Farseer 3.5)