Connecting to system ground for equipment ground
Yes, in 2014 the Retrofit Ground rules were greatly liberalized to allow exactly what you're talking about.  Retrofitting a ground to the Grounding Electrode Conductor (the run between panel and ground rods/Ufer/water main clamp) is specifically allowed.
You cannot sever or cut the Grounding Electrode Conductor. You will need to clamp onto it with a split bolt etc.