You can add an outlet to a light fixture using the following steps, 1. Turn the power supply off to the light switch. Remove the switch plate and unscrew the switch from the outlet box. Pull the switch from the box. 2. Use a voltage sensor to ensure that the power of the box is off. Disconnect the two wires attached to the switch. 3. Remove the existing outlet box and replace it with a double outlet box. Remove the wire nut connecting the two neutral wires in the back of the box and add a third wire to the mix. 4. Reattach the two short black wires and the white wire. 5. Once all the wires are attached, press the switch and outlet into the new box.