adding a ground wire to a 2-wire breaker panel
Since this is your main panel, you can land the ground wires on the same bar as the neutral, as this is the point where they come together anyway.  However, I don't believe you can "extend" the grounding bar using the cutoff bare wire from the panel, as it doesn't fall under anything in 250.130(C) -- if it were connected to a grounding electrode, it'd work though as it'd be a grounding electrode conductor then.