If you are implying a spell to transform you into a "harpy" then we will be unable to aid you. Magick has limits, one of those being able to change your species or make altercations to your DNA.
You could always incorprate harpies into your magical practices, though. Perhaps you could make offerings to them, as the meaning of the word "harpy" is "one that snatches." In greek mythology they are frequently noted for stealing food, so perhaps an offering of food would be suitable for them.