Flex steel conduit makes for a lousy ground. I wouldn't use it for that even it if was allowed. The house I grew up in has that, and corrosion over the years just makes it totally useless.
I'd run a ground back to the main panel. I do not think it will pass current codes without one. You still need the ground rods on the outbuilding. The grounding is for primarily lightning surge control. The actual ground wire provides a safety path for tripping the fuse or CB...a ground rod is not sufficient for that.