From what I know, if your iTunes has all the songs already loaded into its memory, leaving Manage Music Manually unchecked should allow iTunes to place all of your songs into the iPod Touch (If there is enough room).
*edit*
After seeing your second post, if you have deleted the songs from iTunes, the next sync will delete them from the iPod as well. You can copy all the songs on your iPod (not touch or iPhone) by enabling hidden folders in view and going to <iPod Drive>:/iPod Control/Music/
There will be several folders named F0# and those have every song in them.
Copy that into a folder on your drive and add them to itunes. Consolidate them and they will be renamed from things such as XLZO to the actual song name.
The iPod touch and iPhone don't allow disk mode however so you can't use this trick with them.
Last edited by Drakaji (2007-12-05 18:40:02)