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.