yes, you can as long as your airline has a reciprocal agreement with UA, NW, CO, US, etc. There has to be an open seat in the back. I believe UA doesn't allow J/s to sit in first class on 3 cabin a/c but they will sometimes. Make sure you give yourself plenty of time to pay for departure taxes/fees and also to deal with the unexpected. Have plan "B", "C" and "D" just in case.