A system for conditioning a working fluid in a combustor includes a primary combustion chamber, a liner circumferentially surrounding the primary combustion chamber, and a primary nozzle in fluid communication with the primary combustion chamber. A secondary combustion chamber located outside of the primary combustion chamber includes a shroud that defines a fluid passage, a secondary nozzle, and means for igniting fuel in the secondary combustion chamber. A method for conditioning a working fluid in a combustor includes flowing the working fluid through a primary combustion chamber and flowing at least a portion of the working fluid through a secondary combustion chamber located outside of the primary combustion chamber. The method further includes flowing a fuel through the secondary combustion chamber, combusting the fuel, and flowing the combustion gases from the secondary combustion chamber into the primary combustion chamber.