So I think you are going about this the wrong way. Simply network together the computers that have music, and connect you iPhone to the one machine that DOES allow you to sync music. Then, mount the network drives from the PC and other machines on the computer that syncs to the phone, and simply drag over to iTunes on the sync machine the songs you want. NOTE that this may copy the songs from the other computers to yours.
On your iPod, you may be able to a program like Phone View (
http://ecamm.com/mac/phoneview/) (or one of many other product that let you get at the contents of the HD on the iPod) to move over the music to a computer. But again, since you put the music on the iPod, it must be backed up to a computer somewhere. You could always sync the iPod to the same copy of iTunes and then all the music will be on that copy of iTunes as well, and then you can drag it back to the iPhone.
Currently, I have music on two Macs and a PC, and then a third Mac that is my sync machine for my iPhone. Music from all three machines are added to the iPhone simply my mounting the remote drives as shares to my Mac, and then dragging the music from those machines into a folder I have labeled "For My iPhone". In the Sync settings, I have For My iPhone set as the folder that syncs all content to the phone, and behold, all the music I want on my iPhone.
And yes, ALL my music is MP3 files.
