A simple mod like this can be effected in any Gerber editor program. You can also import all the Gerber files into EasyPC and do the modification there and regenerate the full set of Gerbers too. But, depending on the exact detail of the pcb design Easy PC may not import all design features of the original pcb layout. For example, some copper flood areas and pad shapes may have to be re-checked afterwards (on both Gerber Import and Intelligent Gerber Import), depending on the original source program used to generate the Gerber files.
The "best" way depends on a number of factors (as usual). Without seeing the design it's difficult to advise. But from what you are saying a simple mod in a Gerber Editor should do the trick, or just ask your pcb manufacturer to add the copper track for you.