If you are bottling, you can just add it the same way you will add the priming sugar. Remember CO2 is heavier than air, so even if you open the lid of a fermenting bucket, the bucket will 'hold' the co2 inside (providing minimal air turbulence). Also, if you are bottling, you will already be introducing a bit of oxygen to the top layer, which is just a product of the process. It's not much to worry about since it's going right into the bottle and being sealed.
If you are in a carboy, even better. Those do a great job of holding the CO2. You can just pop out the rubber stopper & airlock and pour it. You may have to be creative with mixing it while not disturbing any sediment/yeast that have fallen out depending on how long you have been in secondary. Cheers!