Wiring in unfinished garage

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.

Ok, Class 1 wiring systems NEC article 725.49 (A) sizes and use and (B) Insulation which give you approved insulation for sizes 18 and 16. If it is 16 or larger you need to use Article 310. I would forget the 16 and use 14 nm instead. If you decide on 18 or 16 the equivalent to thhn would be tffn. Or I would check with whomever the manufacturer is and find out what they suggest.

Hope this helps