Thanks for the photo.
It looks like the black wire grounds the Bell box to a bolt on a plate at the top of the meter box. The plate is connected to the conduit and connected to the box with a clamping screw. With all that paint I wonder how good the connections are.
It looks like loosening the nut over the black wire will not disturb the electrical system ground, but it will disconnect the Bell ground. I don't know if that would cause a problem. What does the orange tag on the black wire say?
If you want to stack two wires on the same bolt you might put a washer between the two.
As an alternative, you can leave the Bell wire in place, scrape some black insulation off the Bell wire, and attach your new ground wire to the bare section of Bell wire with a split bolt.
they come in different sizes
http://www.homedepot.ca/product/8str...ge-of-2/910183
I'm not an electrician and don't know your local regulations, so take my advice for what it is worth without a guarantee.