What about this. You run a 4 conductor teck cable and use one conductor as the transformer ground. What do you do with the bonding conductor in the cable.
This would be something you would run into when you upgrade a 208V sub panel to a 600V feed with a step down transformer and use the existing cable.
I would guess you would ground one one of the bond conductor and isolate the other?
A Ground wire must be green or bare, not black or any other color in an ACWU or teck. Are there other stores nearby with a transformer? If it is 600 volt distribution then there must other transformers. Where do their grounds come from? Was the wire you found in the pipe a ground or bonding? If for example the original feeder was EMT all the way from the supply, then it likely that it is also the bonding conductor. The green wire in the pipe could be the ground but you'd need to see where it is connected and how.
If the original feeder was part pvc and part emt then you are missing a wire (the ground) look in the ceiling. Is there a big copper wire strapped up that looks like it goes past each store and has a bare or insulated copper wire hypressed or thermit welded down to the stores individual transformer. Ground wires are not required to follow the feeder so this could be coming from a less obvious route.
To use a cable to feed your transformer you are going to have to add a Ground wire that is continuous from the main service electrode to the transformer or panel neutral. Unless you can purchase an ISO ground cable or get permission from your AHJ then you can't ground with the bonding conductor in the cable or unused Other wire. From experience ISO (isolated ground) in a cable is pretty rare once over #12.
Now for another possibility if you are in a strip mall IE every store is on grade or you store is at grade you could install you own rods or plate but then you have to ensure this new electrode gets bonded to the other electrode. That can be done via the bonding wire in the feeder cable and a regular ground from the neutral of the new service to the plate or rods.
All this can be done in pipe but cables just don't have ground and bonding conductors inside them.