What should I do with the ground wire from my light fixture when there's no ground in the box?
This wire is usually connected to a bare wire ground if available, if not available and the box is metal connect it to the box. If you do not have an equipment ground the wire is not used.

Current code allows a ground to be added as long as it comes from the same panel that feeds this circuit (it can come from another circuit if grounded and fed from the same panel)