iTunes will open no matter what, as far as I know. However, iTunes will ask you if you want to sync the iPod to this new computer. Just say no, and you'll be free to use Senuti. Should work pretty well. Just make sure your iPod is up to date with the newest iTunes/iPod software, and that you're using the newest Senuti.
Your playlists and ratings, however, won't come through. I don't know of a way to keep those.
Also, Senuti has some preferences as to where it puts the new files, and whether or not it adds the songs to your iPod library. If your iTunes preferences are set to copy songs to the iTunes library, you should have no problems.