How do I add an outlet to the end of a circuit that terminates with a recessed light fixture?
Two options:

1) Replace the ceiling fixture with a pull chain model, plus an outlet.  Bypass the present switch.

2) Run a new cable from the outlet to the switch box.  Your picture shows white color NM cable, so presumably it is 14/2 (black, white, ground, 15 amp maximum). Obtain more wire, and hook three wires (black, white and ground) into the respective positions in the couplers.

Read up on basic electrical safety before attempting any of this.  The bottom bundle, as you wrote, is the incoming hot.