Can I add a song to a burned CD?

I am all but sure that this cannot be done.

usually when you make a CD it gets finalized after it's being burned, so the playback will stop with the end of the last track.
You could make un-finalized CDs with Roxio Toast, but that a whole different story.

It is possible to create multi-session CDs in Mac OS X using Disk Utility. See this article:

http://docs.info.apple.com/article.html?path=DiskUtility/10.5/en/duh1749.html

However, this may not - probably does not - apply to iTunes.

It is possible to create multi-session CDs in Mac OS X using Disk Utility

Of course this applies to data CDs, not audio CDs. (The OP doesn't state which he was making.)

For audio CDs, it cannot be done.

The process of burning an audio CD includes a section called "Table Of Contents" that refers to the other songs and is burned onto the CD at the same time. So once the burn is done, there is no way to add more songs and still have a playable audio CD.