Integrating the control of accessory devices into a wheelchair joystick can both optimize function and improve aesthetics. Although the number of commercial systems having this feature is increasing, there is still a number of joystick controllers in the field that do not provide integrated accessory device control. This paper describes the design of an add-on system that allows the user to control both the wheelchair and accessory functions using a proportional joystick controller. The mode of operation is chosen with an external switch that can be mounted at any desired location. Accessory control can include both wheelchair functions, such as power recline, and external functions, such as an environmental control unit. The design is especially well suited as a retrofit to existing wheelchair control systems. A case report of an individual with quadriplegia is presented demonstrating a successful implementation of this system.