Motorized designs of stair-climbing wheelchairs available in western countries are heavy and expensive, and hence, unsuitable for developing countries. Manually operated solutions for stair-climbing wheelchairs also tend to be bulky and complex. As stair-climbing is an occasional activity for wheelchair users, having a built-in stair-climbing mechanism results in complexity and redundancy. In this work, an add-on device is envisaged, which requires the wheelchair to be mounted onto the add-on only when stair-climbing is needed. This work developed a Hex-wheel mechanism to address the demerits of the Y-wheel mechanism commonly used in load carriers, as well as to improve usability for stair-climbing. Furthermore, a suitably designed actuation mechanism was applied to the Hex-wheel to enable manual operation. Finally, a prototype of the stair-climbing add-on device was built to validate the developed mechanisms. The force required to operate the device was measured and found to be within 10% of the predicted theoretical value. The novel design provides a solution manually operable by an assistant, which is cost-effective and independent of wheelchair type to improve accessibility in low-resource settings.