Hi,
Yes you can wear over your existing hair without shaving,providing your existing hair is not to thick,where you are going to locate the piece.
I used blue liner tape and attached over my existing hair,pushing the piece down onto my scalp to make sure the tape stuck,that way i was able to just wear on weekends to try everything out before i fully commited and shaved my head.The blue liner was actually sticking to my scalp through my thinning hair,it works well.I just used plenty of alcohol to release the piece,that way it didn't pull any existing hair out,you can then go back to your existing hair between wearing.

Regards Topcat