I have a lot of frozen stock and need to free up freezer space. Can I thaw and then can this stock in a pressure cooker?
If it was frozen immediately or quickly after coming off the heat, then it's not much different from "fresh" stock. Food keeps indefinitely in the freezer, safety-wise, and the quality isn't going to be impacted very much because it's mostly water to begin with.
Just don't can any meat products that have been exposed to room temperatures for more than 2 hours, and follow the same safe canning practices (i.e. pressure-canning) that you would for any low-acid product.
The Food and Safety Inspection Service division of the USDA has this to say about refreezing foods:
Do not refreeze any foods left outside the refrigerator longer than 2 hours; 1 hour in temperatures above 90 °F.
I would apply the same rule of thumb to canning previously-frozen stocks, as the purpose of canning is largely the same as freezing--to preserve food for long periods of time. The only difference is that freezing attempts to halt biological growth by suspending water molecules in a solid state, which prevents most biological life from reproducing. Canning attempts to halt biological growth by removing/limiting the amount of oxygen available.