Waze currently does not have any interface to add, change, or remove gas station locations. The USA is the 'test' country and Waze imported the station data from an external source to get data into the system. It's a large task to create an agreement for this data in every country world-wide.
It seems that Waze will be taking another approach when extending to additional countries. I believe that Wazers will be responsible for all if this data. We need an interface to maintain the data first.
Wazer in San Diego, California USA and Tijuana, Mexico
iPhone 5, iOS 7.1, Waze 3.7.8