If you install something that lets Windows read your HFS+ partition, such as MacDrive, then you should be able to add the songs to your iTunes:Windows Library. I think. You won't be able to copy your other library data over (playcount, star rating, etc.) because I think the formats are different.
Well.......... that would be if you're using Boot Camp. If you're using Parallels, I think you can just create a share folder or something? I haven't used Parallels lately so I can't remember.