A system for supplying a working fluid to a combustor includes a fuel nozzle, a combustion chamber disposed downstream from the fuel nozzle, an inner flow sleeve that circumferentially surrounds the combustion chamber and a plurality of injectors circumferentially arranged around the inner flow sleeve. The plurality of injectors provide for fluid communication through the inner flow sleeve and into the combustion chamber downstream from the fuel nozzle. The system further includes an outer air shield that defines an injection air plenum that surrounds the plurality of injectors. An inlet passage extends through the outer air shield to define a flow path into the injection air plenum. An outer sleeve is slidingly engaged with the outer air shield. The outer sleeve has a first position that restricts flow through the inlet passage and a second position that increases flow through the inlet passage.