I just don't know of any legal way that you can do this. If you had three insulated conductors and there was no other metallic connection (water pipes, etc.) between the feed location and the structure where the sub panel is located, there could be some exceptions. A grounding conductor in a piece of 10/2 would not be considered to be an insulated conductor.
I don't even see how you could do this with a transformer while still maintaining the original circuit to the irrigation pump.
Sorry for the bad news, but it looks like the original UF is going to have to be replaced.
