We report on the experimental observation of a new resonant mode in a spatially modulated annular Josephson junction with a trapped chain of solitons. We observed voltage steps in the current-voltage characteristics of the junction. These steps appear when the spatial periods of the soliton chain and the junction modulation are incommensurate. The effect is explained in terms of the motion of defects (supersolitons) along the pinned soliton chain, which were found earlier iin numerical simulations by Ustinov [Phys. Lett. A 136 (1989) 155].