Home sharing is only going to work through iTunes.
You do not need to buy music from iTunes. It will happily import your current music. You can either let it duplicate it in it's own library, or have it leave the music where it is.
First go to the Advanced tab in iTune's Preferences and choose whether you want it to try and organize your music and/or copy the music to it's own folders. I you say no to both it will leave your current folders untouched, so that you don't have to worry about your original music files. That will probably cost your some neatness in the way iTunes presents the music.
Then go to the Files menu and choose add library. Navigate to the folder with you music and add it. I believe this method will leave you with whatever organization you had in the folders in iTunes. But now you'll be able to share and play your music.
Probably, anyway. Can't say I've done this myself, so no guarantee. But the method doesn't effect your original music, so even if it doesn't work satisfactorily, all you've lost is some time.
Added Note: There are other solutions to stream you music from you computer to iPad. Home sharing is probably the easiest way to do things if you already use iTunes for your media, but a little research on your part might net you a method you'll like better.
