I would say yes. The Massoth S comes prewired with brown and white track power connectors. You can just plug those onto the brown (bn) and white (ws) posts on the 55021. Set the address to be the same, and makes sure the speed settings CV 2, 3, 4, 5 and 29 are the same on the 55021 and S decoders and you should be fine. It might be better to replace the 55021 with an LS decoder which has motor and sound integrated, but more expensive and probably soldering required for the lighting. If you're happy with the 55021 then adding an S decoder is the easiest solution.