It depends on the UPS.
Many of the smaller ones run off of a internal 12 volt battery system so its just a matter of adding external connections to one so it can be connected to a larger external battery source. Bigger units will either use 18, 24, 36, 42, 48, or 60 volts as the battery source.
However the continuous output power capacity of the cheap UPS units is only around half of the rated capacity when used for extended periods.
Normally they are sized so that when fully loaded the internal battery went dead the same time the electronics reach their maximum working temperature. So by using an external battery source of larger capacity it is possible to burn one up if ran to long at a higher load while being under its rated capacity.