Would it be possible for me to use a regular store bought pump to tap a keg, then untap it and place it in a kegerator with a co2 pump without the keg going flat? Would I be able to change out the oxygen that gets put into the keg with co2, or would I be spoiling the keg much quicker because I use the regular pump? Thanks.
If you plan to consume all of the contents of the keg within a couple of days to a week, this may be a possibility. Using oxygen to tap a keg of beer causes the beer to stale exponentially quicker than regular CO2, even more so in cold temperatures. I would advise against doing this unless you plan to have it all drank in one sitting at a party.
Depending on the style of keg, you may consider purchasing a portable CO2 charger. This one is a good option if you have cornelius kegs. Go easy on the trigger though, as it burns through CO2 quickly, and there's no regulator on it. If anything, plan to pour out slowly from the keg, maybe even over-carbonate a very slight bit more than normal to compensate.