In this study, we propose a method for electric wheelchairs to climb steps using a passive add-on mechanism. The mechanism utilizes the rider's postural shift motion and canes to enable independent step climbing without the need for caregiver support. In this mechanism, auxiliary wheels are affixed behind the rear wheels of the wheelchair via a hinge joint. Step climbing is accomplished by 1) elevating the front wheels using the canes, followed by 2) raising the rear wheels onto the step using the spring force stored in the addon mechanism and the rider's postural shift movement. To optimize the link lengths and springs of the add-on mechanism, we developed a quasi-static step-climbing model. Subsequently, we fabricated and tested a prototype of the add-on mechanism to validate its capability to ascend steps using the user's body power and postural shift. The experimental results confirm the system's ability to ascend a step approximately 23 cm in height within 15 seconds.