The songs in the ipod touch import from another music library, how to sync ipod touch with itune without delete the songs in the ipod

I have import songs from a computer to the ipod touch through itunes. Then I try to add more songs from another computer also have itune. The itune from the later computer send me a warning message that the songs in the Ipod touch came from a different library and the songs in the ipod will be deleted when try to sync wth the computer.

Please advise how to sync with the computer that I can keep all songs in the computer and ipod