As others said, only use what came with your UPS. The APC Back-Ups line is a cost-efficient design that is mean to be cheap and affordable. It does not have the heat dissipation ability for prolonged battery use. Chances are even if you *could* get the unit to read and charge larger batteries, the unit would end up failing under prolonged battery charging and battery use. Not worth the risk. If you need additional capacity, you should purchase a unit that has extendable battery banks. Those tend to be costlier than standard units.
For instance, specifically for APC, you can find the Back-Ups Pro 1500VA version of your UPS under BR1500G-IN. This model is specifically designed to have a supplemental battery pack installed. It costs about 60% more compared your unit.