5v usb cable to power a 2AA Kitchen Scale
First off there is definitely something wrong with the kitchen scale if it is truly using up the batteries every few days. No reputable manufacturer would release a product that worked like that. Whatever the problem is I would question whether the unit is even accurate anymore. So in the long run you should really consider simply replacing the unit with one that does not consume batteries like this. 

Now about your battery eliminator idea. DO NOT DO THIS AS YOU HAVE PROPOSED. The unit was clearly designed to operate at approximately 3V. You should not attach a 5V supply to it. Now since we have already established that the unit is already broken in some way if you increase the voltage from 3V to 5V the current draw from the source will most likely be even more than the batteries are getting loaded with. This could lead to further damage in the scale or overload the charger unit. And if the charger is some very cheap import they often die in dangerous ways if they are overloaded.