Although being a citizen of Lego Land (well almost, but we're both Danish), I don't know if the NXT is supposed to do that and if it ain't, it would take some serious hacking to do it (i.e. forget it).
If you have a link to a specsheet on the NXT, please post it and someone will likely review it and tell you if it's possible or not.
I do have a feeling that it would take a rewrite of the firmware in the controller, as well as mounting sensors on the motor for detecting it's position or number of revs.
If you want to be able to control stuff completely, you better build something from the ground up.