Copy the tracks you want from one machine to the other and put them in iTunes.
When you have all the required tracks in iTunes, make a playlist, sync the iPhone and make sure that the new playlist is included in the sync.
I have a ipod with an older version of itunes on one PC and an iphone with a current itunes version on another PC. I would like to get all my music from the ipod onto the iphone. Any way of doing this?