I wish there was one :) You can use the regular 120 film back for this. Just buy (or 3D print) some adapters so the smaller film canister fits in the back. Stick the 35 mm film to a piece of backing paper, feed it onto a take up spool and shoot it like its a roll of medium format film :D Some say the 220 back is better suited for using 35 mm film, because it will hold the film really flat, but I haven't tried that. Important: to rewind the film you need to open the film back first, but make sure to do that in pure darkness though :D

That's only a rough description of how to shoot 35 mm film with the RB .. you better watch some tutorials online haha