I believe the 737-900ER and all the MAX variants have 26,035 litres fuel capacity, the wing and centre tanks are the same in all cases. Auxiliary tanks such as on the BBJ may be an option, but are pretty well useless for airlines such as WS, as they will be trading payload and seat capacity for range with auxiliary tanks, so will have rapidly increasing CASM. Even the standard full fuel capacity is difficult to use with a full flight and typical baggage / cargo load, as MTOW will be reached before a max range fuel load can be used.