I have knob and tube wiring for large portions of my 1950's house, including to my bathroom. Currently, I have hot and neutral coming in from the knob and tube wiring and a separate ground wire coming in from somewhere(we haven't had any electrical work done since 2014, so I don't know how that was permissible. I can't seem to trace the wire in the wall). This powers one 20A labeled GFCI (without a separate prong for a 20A plug?) and a light. The circuit is on a 20A breaker. I would like to add a second outlet by adding some NM cable to the load terminals from the GFCI (and the existing ground connection).
My jurisdiction follows the Residential Code of Ohio. Would adding this outlet be safe, and would it be permissible under code? I'm afraid of violating restrictions on extending circuits involving grandfathered wiring.
Here is a photo of the GFCI box. The nm cable goes out of the box to a switch and light. The power comes in via the old knob and tube wire on the side.
Here is a photo of the knob and tube wiring that feeds this. No idea about the NM that supplies the ground.