I think you can use WMP to manage your iPod, though I wouldn't swear to it - I'm sure someone else will tell you if I'm wrong. The incompatibility is in the default ripping format: iPods won't play the WMA format, but all your tunes are ripped in MP3, so you have no issue there as iTunes plays MP3s; and so does the iPod.
You don't have to use iTunes to manage your iPod, there are plenty of third-party applications around, some free, some not. I've used Xplay click here and liked it very much as it gives you the drag-and-drop capability and doesn't impose the restrictions on being able to transfer music from one PC to another via the iPod.
I also like iTunes, barring my irritation that Apple has yet to bestir itself to make it play nicely with Vista - it's a bit hit and miss at the moment. If you do decide to use iTunes, it's easy to import your music. You just simply click (off the top of my head) Add folder to library and browse to My Music or wherever you've stashed your music. If your tags are tidy, iTunes will read them with no problems.