How to add cable to outlet box?
This is fishing. You can usually slice a little bit of drywall from the area above the box and stick an electrical snake in there and try to snag the cable and direct it over to one of the access points on the box. You can also loop the cable before running it in the wall and then try snagging it through one of the access points into the box. Sometimes you just have to cut the the nails with a hacksaw blade, remove the box and then get a new "old work" box that is supported by drywall clamps. Pull the cables into the box and then install it in the same hole as the old one.