Need advice if this wiring is okay (adding outlet into a run)
I can't tell if you are in cable or conduit.   Cable is a stiff but flexible plastic jacket with several wires very tightly wrapped.  Conduit is plastic or metal pipe  with wires loosely added. 

If you have an inline conduit, and you want to add a box, that is easily done.  However you have to pull out the wires before you begin.  When you put the wires back in, you will find they are too short for all you need to do, so you will need to either replace parts of the wire runs, or have the existing wire-run run right through your new box without stopping, then add an additional set of wires to the next box up or down.  I use this "run thru and follow it" technique when a box is getting crowded.  If your existing conduit is metal, stay in metal. 

If you have cable, then you need to install 2 boxes, make good use of the existing cable run, and install new cable between the 2 boxes. Alternately you can replace all the cable between the new box and the next existing box down the line.  

The reason for this in both cases is that you need to have considerable length of wire at the box.  You cannot "runt" it with an inch of slack on each wire and the wire nuts crammed into the back of the box.  What you want is 9-10" of length on each wire. The legal bare minimum is 6" past the end of the cable clamp or pipe, plus 3" beyond the wall surface, but the legal minimum is cramped to work in.