Can a 2 gang surface mount box be mounted over a 1 gang recessed box?

Here's what I would do. Buy a plastic 2 gang old work box. Use a hacksaw blade to cut through the two nails that hold the existing box to the stud. Next, remove the wires from the existing receptacle and remove the box. Some will break apart using a pair of pliers. Once the box is removed, use the two gang box to trace the additional drywall that needs to be removed. Cut away as needed. Place the wire in the box and install the box in the hole. Tighten the wings on the new box., Then use a couple of drywall screws on the inside of the box that the stud is next to. This will make the box more secure. After that, it's just a matter of rewiring the old receptacle and adding jumpers from it to the new one. Don't forget to buy a two gang cover for the new box as well.

Surface Mount Electrical Box

2 Gang Electrical Box

Yes there is a surface box made for just that purpose. There is a scribed knock-out in the back plate that lines up with the screws in a single outlet box. One brand is "Wiremold" and can be purchased at Home Depot or most home improvement stores.

There is a box designed for that http://www.amazon.com/gp/product/B001M5E9OU/ref=as...

More common is a handy box size, http://www.amazon.com/gp/product/B000HEKS5Q/ref=as... , if you go this route I recommend you only use the box as a splice box.

If you put a 2 gang box and just threat the wires through a knock out with a bushing I think most inspectors would consider the interior box inaccessible if you run conduits out of the front box.

Can you mount a 2x4 deep box extension on the front of it? That will only hold the single duplex rec, but it will project far enough to add conduit and continue down the wall, it'll be simple to install and it'll be more sturdy

Yes, you just screw the 2 gang into the wall typically, rather than the 1 gang. If the mounting holes line up, then use those. If its outdoors, make sure you but the outdoor model with seals.