Unless your area hasn't adopted NEC2014 or later, if there's no other defect to be corrected except a mising ground wire, you can simply run a 12AWG ground wire (green or bare, or green with yellow stripe) from the box (connected to the metal box with the box ground screw) to any of:

• Ground at the panel feeding the box.
• Ground of another circuit with 12AWG or larger ground, fed from the same panel.
• The Grounding Electrode Conductor connecting the panel to the ground electrodes

You should verify that all the connections are properly tight on the conduit fittings; you may opt to simply run a ground wire in the conduit as well as grounding the conduit. Grounding the conduit is not optional, running an additional ground wire inside it is optional.

Unless your area hasn't adopted NEC2014 or later, if there's no other defect to be corrected except a missing ground wire, you can simply run a 12AWG ground wire (green or bare, or green with yellow stripe) from the box (connected to the metal box with the box ground screw) to any of:

• Ground at the panel feeding the box.
• Ground of another circuit with 12AWG or larger ground, fed from the same panel.
• The Grounding Electrode Conductor connecting the panel to the ground electrodes

You should verify that all the connections are properly tight on the conduit fittings; you may opt to simply run a ground wire in the conduit as well as grounding the conduit. Grounding the conduit is not optional, running an additional ground wire inside it is optional.