I seen few entries at instructables related to modification of existing UPS devices which explained via dangerous way. in some cases purchasing an used device might be more smart choice. but if you already have a lot of batteries, this entry shows how to use them without losing an eye or arm.
you will need:
+ plastic gloves
+ hot silicon stick
+ at least 1m cable which can handle high current
+ extra batteries which is same voltage & same capacity
Step 1: Preparing the Device
you need to wear plastic gloves first, it's capacitors can hold the electricity upto 24 hours which is still dangerous even if it's not connected to any power source
+ unpack it
+ remove the existing batteries
+ cut the heads of battery connectors
+ extend the battery cable
Step 2: Extend the Battery Cable
+ drill holes for previously extended cable
+ fit the cable from hole and fill the surrounding area with silicon. that's prevents the cable damage while moving the device.
Step 3: Connect Batteries in Parallel
if your ups uses single battery, you can just connect them in parallel. if it uses double battery, you have to consider two of them as a single while adding extra banks of batteries.
for example, my ups was using two 12v 7ah serial batteries which is makes 24v with 7ah capacity. if you attach another two in parallel that produces 24v with 14ah capacity.
you can add unlimited battery banks if they're match same voltage and capacity.
Step 4: Usage Notes
+ usually this kind of devices designed to work about 5-10 minutes, after 20th minute the voltage inverter coil and mosfet's getting hot even can melt the device's case. you have to install cooling fans if your ups made from plastic case.
+ you have to disconnect all batteries from each other when you're turning it off. while ups is active it's keeping the batteries alive by feeding them with "float voltage" (https://en.wikipedia.org/wiki/Float_voltage) otherwise they're try to charge themselves without ups connection and quickly die within few days.
+ DO NOT USE lead-acid (used in car-marine) batteries if you're using it indoor. these kind of batteries produces poisonous gases and there is risk of explosion on high loads. take serious that, i learned via hard way.