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So, we've ran into a couple folks who have annoying 3.5" octagon or pancake boxes firmly embedded into exterior walls, and want to mount a receptacle in them -- the pancake box from a linked thread is depicted below, along with a dimensional drawing of same.

annoying 3.5" pancake box embedded in wall dimensions of said annoying 3.5" pancake box

Unfortunately, the screws provided on a 3.5" octagon or pancake box for device mounting don't fit much of anything besides a light fixture as they are spaced 2.75" apart -- single and duplex receptacle yokes are too big with their 3.28" standard mounting screw spacing, and flanged outlets are too small as their mounting holes are 2" apart, as can be seen below.

duplex receptacle dimensional drawing flanged receptacle dimensions

So, can you use a raised 4" round to 3.5" round adapter mudring, such as a Crouse-Hinds TP332 or the Raco 727 depicted below, mounted with its 3.5" side to the existing box, to provide 4" standard mounting holes which can then be used for a 4" weatherproof faceplate adapter (Taymac PRBA400G, depicted below with a different box) or weatherproof round extension box as well as a faceplate adapter (Bell 5363-0 or equivalent extension box)? (Of course, you'd have to do some extra work to make sure things are sealed appropriately for outdoor use due to the mudring & box sticking out, but that's beyond the scope of this question.)

A 4"-round to 3"-round mudring -- the Raco 727 4" round to weatherproof cover adapter -- the Taymac PRBA400G 4" round weatherproof extension box -- the Bell 5363-0

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    Inverting the mud ring is probably a code violation. They are usually stamped with the cubic inch area added to the box and in some cases have the words "under side" stamped in the metal. I know Blank plates have underside so the sharp edge from the stamping process is facing the correct direction. Looking at the box it appears to be nailed in the back so an "old work" box may be to deep to use. A shallow single gang metal box may work using a nail jack to pull the 2 nails then install the new box.
    – Ed Beal
    Aug 10, 2016 at 13:53
  • So long as the lip is not inside the box (without being protected) it should pass an inspection.
    – Ben Welborn
    Aug 10, 2016 at 15:43
  • @EdBeal in other words "it can, but not for anything electrical" lol.
    – Harper - Reinstate Monica
    Aug 10, 2016 at 19:41
  • Does code allow an extension box with a receptacle in it, hung by a crossbar?
    – Mazura
    Feb 12, 2021 at 23:08
  • @Mazura -- that's a darn good question
    – ThreePhaseEel
    Feb 12, 2021 at 23:27

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Here in Australia you'd probably use a surface mounting block over the top and then an IP rated outlet.

Mounting block is bigger than a round j-box. May need to seal between block and wall and then you're done.

Along these lines:

enter image description here

Source: http://updates.clipsal.com/assets/images/products/A00047211.PNG

Not sure whats available and standards in the US but if surface mounted over the top and larger, shouldn't be an issue but uglier than flushed.

May be able to drill/chase in a large wall box, however it's quite a bit of work.

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