iPod Need advice about using Senuti

I am trying to load up music from an old ipod onto my new imac using senuti. I already have an itunes library set up with lots of music on my new imac. It says when I connect my old ipod in disk mode to the imac that it will erase all of my music on my ipod if I sync it. Do I need to just erase the current itunes library on the new imac and start from scratch by using the old ipod to generate a new itunes library? Please help. This is getting very frustrating. I have 30 Gig of music on my ipod that I do not want to have to reburn all of 800 CD's again.