I had a similar situation. I serendipitously found my iPod I thought I had lost for good a couple days ago. I spent a couple years compiling very rare songs several years back. I wanted a way to consolidate my newly reunited music from my old iPod into my current library.
I don't know if this method is officially sanctioned by Apple, but I used a program called xPort (just do a Google search). It took all the music from my old iPod, stuck it in a folder on my desktop so that you could easily import that folder from within iTunes.