Look at this site:
It gives details of sites where you can transfer music back from you iPod to your PC.
Once you get your music back on your PC, wherever you want to store it, then surely you can transfer it back to iTunes.
I am a new user of iPod and started off with iTunes, but did not like its limitations. I want to put music on my iPod and take it off when I want.
There is a programme called Sharepod which has the facilities you require , although I have never used it. It is free.
In the end, I bought a programme called Anapod Explorer, which I can use quite easily to manage my music.
You can "google" ..transferring music from iPod to PC which is what I did when I was fed up with iTunes.
Good luck, I do sympathise.