I am trying to add an exterior outlet to the rear of my house using an existing circuit that was once wired to a floodlight, and want to make sure I am grounding the circuit properly with my modification, or if it is even necessary. The circuit was cut-off at the exit from the house (prior to my ownership), probably to make room for an AC unit installation (I'll try to add a few pics). At the exterior of the home there is a metal "elbow" that is fed by metal conduit through the exterior wall/siding from the interior. My plan is to run new metal conduit up from the elbow into a new metal gang box with an outdoor in-use GFCI. I have read that you don't need to have a ground wire in a metal conduit, but the metal conduit is not in any way grounded to the main panel, so I am concerned I need to run new wiring that includes a ground wire from the main to the junction box that feeds through to the exterior.
The existing circuit is wired from the main panel into a metal junction box using 2-strand ROMEX (without ground wire), the hot wire is tied to a ROMEX feed that exits the junction to a switch in another room, then the return from the switch is wired to THHN wire that feeds through the metal conduit to the elbow at the exterior of the home (one feed, one return). I'm wondering if I need to rewire to the junction from the main panel with 12/2 ROMEX to properly ground the junction/conduit?
The three pictures (if I figure out how to add them) show 1) the termination of the circuit outside, 2) the wiring of the junction box, and 3) the conduit running from the junction box through to the exterior wall/joist.
I am a mechanical engineer with extensive home/appliance repair experience, understand the basics of home wiring, and have added a new circuit to a home once, but otherwise this is new territory and I haven't been able to find answers to my specific question anywhere so far. Any help is appreciated!