Showerheads capable of attaching to bathtub faucets assume the form of hand-held showers consisting of long, flexible, rope-like pieces that attach directly to a bath faucet. Attachable showerheads range in nature from incredibly basic to complex systems with filters for purifying water and various settings affecting water pressure and pattern. Installing an attachable showerhead allows you to shower in a bathtub without opening a wall or adding new plumbing features to your bathroom.
Whether or not you can attach a showerhead to your bathtub faucet ultimately depends on your faucet. An attachable showerhead requires its own outlet and diverter on a faucet, or else it must fit over the bath spout. If it fits over the spout, you can only take a shower -- unless you remove the showerhead. If you want the option of both, you need a bathtub faucet with a separate spout for the showerhead and a mechanism that switches the water flow between the tub spout and showerhead.
If your bathtub faucet cannot accommodate an attachable showerhead, you can replace the faucet with a set. Sets constitute units made by home appliance and bathroom specialty manufacturers that contain both a bath faucet and a showerhead. You can purchase sets for bathtubs adhered to a wall and bonded to a floor. Sets generally cost significantly more than your average attachable showerhead.
How an individual attachable showerhead works ultimately comes down to the manufacturer. These units, known alternately as attachable showers, attachable showerheads, hand-held showerheads and attachable shower kits, work in numerous ways. Some exist for the express purpose of attaching to existing shower fixtures; others suit specialized types of bathtub faucets; still others attach directly to a valve, which requires opening a wall to access plumbing. Speak with a hardware store employee or contractor to find the right attachable showerhead for your needs.