I would think you would add it one side..and subtract it on the other.
I was looking at this:
http://books.google.com/books?id=3z...&resnum=2&ved=0CB8Q6AEwAQ#v=onepage&q&f=false
Gives an excellent discussion on how to add tornado vectors..probably should throw in the fact that it appear to be multi-vortex from the videos which would have increased the damage potential from the internal suction vortices.
Thanks for pointing out that book. I may have to buy it.
I'm not going to argue the findings and methods of Tom Grazulis, but that doesn't change the fact that the radar measures instantaneous velocities with the tornado's move speed already factored in. Thus, you wouldn't add anything to the wind speed in the image to get a sense of the actual ground relative wind speed.