New junction box, or pigtail within receptacle?
If you have enough wire to have 6" from the back of the junction box where you tie in a junction box would be the easiest way. Most of the time the wires are tight. then there are 2 options. #1 to pull the wire down from the outlet to your box using the wire to pull a string or the new NM cable down to the box some times the easiest is to pull a string when pulling the NM down then use the string to pull the new NM up. #2 Option would be to pull into the existing box from downstairs this can be a challenge getting the NM into the outlet box (if there is enough  room in the box to make the junction). I usually use #1 It cost a few dollars more but saves in time trying to fish wires in a closed wall.