To answer that, you need to understand how the enclosures are tested.
For the NEMA-3R test, a driving rain is sent onto the enclosure at about a 15 degree angle. At the end of the test, there cannot be a 'significant' amount of water in the enclosure.
In addition, there is a rule that factory KO 'pre-punches' cannot be located higher than the lowest energized part.
Some understand the latter rule to mean that ANY opening made above the lowest energized part has use a Myers' hub. I'm not convinced of that logic for vertical faces, though I agree when the opening is on the top face. (After all, snow and water can accumulate on the top).
For openings on protected faces - like the back - I have a bit of trouble imagining how any significant amount of water can ever reach the opening - let alone enter it.
Remember ... NEMA-3R is essentially a glorified umbrella; it's not intended to protect a directed water source, such as a hose stream.
You can use a nipple or conduit in the back bottom factory KO and maintain the rating. If its a concentric KO, and you have any rings left you must bond with a bonding bushing. Not required with PVC of course.