If I need to add liquids in small amounts to a carbonated keg - like flavorings, hops extracts, finings etc etc - all I do is disconnect the keg from the gas, vent it's pressure via the pressure relief valve, then unscrew the relief valve, a quick squirt from a sprayer with starsan, and use a syringe/dropper/turkey baster to squirt the liquid into the hole... More starsan and screw the valve back into place.
No need to open the keg and much less oxygen introduced to the system as well... Quick purge of headspace and no harm done.
TB
