Actually, "growing" (even an unnatural, e.g. a third eye) temporary, functional body part can be done with MuCo Base 5, with T:Part (Magi of Hermes, p.49). This can be a leg, or an arm, or even a pair of arms. A Hermetic prothesis would then be something like MuCo30 (Base 5, +1Touch, +1Conc, +1Part, +5levels item maintains Concentration, +5levels 24 uses/day). Note that it causes warping, so you don't want it to be always on!