I have converted a couple to the GHG motion cone system but don't use them that way very often. You can convert them but still use the legs on them if you want to. The only thing I found is that the GHG stakes are a bit short because the BF is so large.
Cut out the inner circle in the foot attachment part of the body. Use a stick or steel rod to find the balance point and drill a small hole in the back. Insert the motion cone and screw to the body. Note that the motion cone will be a very tight fit going through the hole and you may need the assitance of someone with small hands to hold it in place while you are screwing it to the body.
As you see in the picture the attachment points for the foot assembly are still intact so they can be used. You wouldn't have to use the motion cone. You could simply put a hole in the back and use a motion stake, but you might get some spin and it might be harder to find the hole in the dark that it would be with a shell. If you have the originals and the B-2s I would recomend doing it on the B-2s as the smaller decoy will be less weight on the stake and will give you more stake length to work with.
Another Idea I had was to insert the motion cone then drill a hole through the foot assembly so a plain rod could be inserted without taking the feet off. Again there would be no spin control Unless one welded a T onto the rod that would stop against the legs.