Traditional cost accounting systems are rarely capable to fully support management decision making in organizations. The design of innovative costing systems like Activity Based Costing (ABC) can offer a valuable alternative, however, their design proves to be very challenging. Although the use of business process modeling in cooperation with Activity Based Costing and Activity Based Management (ABM) has been extensively covered in the literature, few articles can be found that refer to the design of a full costing system with the use of a modeling architecture. This paper presents the use of a proposed business process modeling architecture for the design of a full hybrid traditional accounting and Activity Based Costing system of a company operating in the gas industry. The results of the study suggest that business process modeling can enhance the design of a costing system, minimize errors and maximize the acceptance level of all the stakeholders of the system.