Yes, people do it fairly often. especially in the older 36v carts that already have 6v batteries.
Usually mount two more in the bag well and run cables through the access cover. It does make a big difference in the overall range, and can mean better power for climbing hills with the right setup with more amps available from the pack without as much voltage drop under load. It’s a little more expensive, but usually worth it for the longer run times.
Post some pics of your setup if you can.