People with severe disabilities are limited in their options for independent mobility. Amyotrophic Lateral Sclerosis (ALS), Spinal Muscular Atrophy (SMA), Spinal Cord Injury (SCI), cerebral palsy, multiple sclerosis and muscular dystrophy etc., seriously affect the extremities and make the operation of commercially available wheelchairs and Powered Wheelchairs (PWs) through a joystick User Interface (UI) difficult or near impossible. Symptoms affecting individuals can vary in severity and consistency, reducing the effectiveness of singular control modalities. This paper presents Smart Wheelchair Assistant for Autonomous Navigation (SWAAN), an add-on-device for existing PW platforms that provides an alternative set of Human-Machine Interface (HMI) modalities for user selection. SWAAN offers users control through either speech recognition, teleoperation or autonomous navigation, beneficial to people with different levels of disabilities. This addresses the constrained application of singular control modalities and minimises the physical and cognitive effort required for Activities of Daily Life (ADL). SWAAN demonstrates successful autonomous navigation through outdoor urban environments at The University of Sydney (USYD).