Assistive devices and exoskeletons have critical importance for people with manipulative and locomotive disabilities. One of the major purposes of such devices when used for lower extremities is to help provide the postural or gait stability of the user. However, current lower extremity exoskeletons available lack the sufficient foot support area to guarantee a safe operation for the rehabilitation of patients, and normal posture/gait for users carrying heavy loads on backpack. As a result, these devices may require an intensive control effort to supply the posture or gait stability and can demand additional therapist help during rehabilitation. In this paper, we proposed a novel adaptive foot system to enhance the required stability of lower extremity exoskeletons as an add-on device. The method essentially aims to automatically extend the support area behind the heel during walking. The proposed adaptive foot system can extend passively during stance and retract during the toe rocker phase, which allows increased support areas during stance and prevent collisions to the level ground during swing. It is practical to implement and can be employed without necessitating an actuation power. The proposed wearable system will particularly be valuable in rehabilitation for enhancing the stability where safety of patients is particularly critical. It is also anticipated that the system can be a complementary device for current exoskeletons or humanoid robots to enhance their stability. A detailed description and numerical analysis of the stability in sagittal plane is presented for postural and gait cases in this paper. Experiments have been also conducted to prove the effectiveness of the adaptive wearable device for postural and gait stability.