You can increase the capacity of the batteries from the original equipment, but you cannot put more load onto the UPS. Your runtime may be increased, but you may have trouble fitting the car batteries into the case. Some UPS's have external connectors for external battery packs. Find one of those and add the batteries to it. Make sure that you maintain the DC voltage which the UPS requires. It may be 12 volts or 24volts. Two 12 volt batteries in Series.
You can go here to replace these batteries: http://www.batteryspec.com/ecomm/cart.cgi?action=link&product=19210
Unfortunately not. The batteries for UPS's are made exactly for each model, not only in voltage and amperage ratings, but also to fit in the UPS casing.
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