How to burn a song onto a CD that already has songs on it?

if you are holding a CD that you can play in any CD player like a walkman car or your home stereo then it cannot be changed. it is a permanent CD. if it says CD-rw (rewritable) on it, the CD can be appended in a CD authoring program, but will only be playable by a computer's optical drives. unless you finalize the CD which allows it to be played by most non-computer CD players.