In the main service panel there are 2 bus bars ( neutral and ground ). I
don't have any screw holes left to add an additional branch circuit to a
detached subpanel located 60' away from the main. Can I:
1. Add an additional bus bar to the main
panel and if so, how do I connect it to the old bus bar(s)? or
2. Replace the entire full bus bar with a
new longer one, or
3. Neither of the above 2 can be done at
all???? If this is so, what can I do to solve this problem?

In the subpanel, the bus bar that is away from the breaker panel itself that
I added to the subpanel box, does that bus bar become the ground bar or the
neutral bar? I think it's the ground bar because it IS separate from the
rest ( not bonded ). Is this the right thinking on that?