I'm sure that you have neutral and ground in the sub panel. (If not it's simple to pull a wire through the conduit.) Now the next question, as I cannot see it in your picture, is the panel direct wired into the sub panel or does it plug into an outlet?
I'd like to know but it doesn't matter as I'll tell you about a fix that you can do. Buy 25' (or whatever length you require (+ a little) of Flexo PET Expandable Sleeving
in whatever color you wish. You need to get the product FW050 which is 1/2" sleeve. That will set you back $12, a whole lot less than replacing the cable. You will also need to get a length of stranded
copper wire for your ground conductor. Lowe's, HomeDepot or Ace Hardware should have it in stock by the foot. I believe #10 would be ideal but #12 would be adequate as well.
Once you have both pieces, disconnect one end of the cable, run your ground wire parallel and then sleeve it. I'd suggest that you use a piece of HS365 Shrink Tube
on the ends to secure the sleeving in place. Once that's done, reconnect the cable and you are good to go.
I sure hope this helps you. Please let me know.