Yep, this is a downside to the Itunes software. If you want to plug your IPOD into another computer it will erase all songs on the IPOD first, and than allow you to transfer or build playlists on the other computer.
You could try getting a copy of another song management software called X-Play. This allows you to transfer songs from computer to computer without deleting or replacing your IPOD songs. Also, you can try loading the MP3 files on your IPOD using the DATA portion. The IPOD Shuffle has the ability to set aside a certain portion of it's memory to use as data files. You may be able to load the songs on their that way.