I'm going to do the exact same thing wednesday, only I'm going to add about 18" of cable.
The way that I read and understand [250.140 Exception], I'm positive (in my own mind) that it's code compliant. Other's opinion will vary.
The circuit is existing, I'm only going to extend it.
I've done it before and the inspector didn't have a problem with it at that time. Who knows what the next one will say?
The original location had a electric range. At a later time, the unit was replaced with a gas range and the installer used the existing 6/2 SE w/ground to supply the 120V receptacle needed for the gas range. In doing so, he cut off about 18" of the SE, installed a JB in the crawlspace (directly under the range) and spliced on #12 NM to feed the receptacle . All I'm going to do is put it back like it was originally (not counting the JB and splices).
Whom ever installed the gas range spliced (in the JB) both the grounding and grounded conductor of the NM to the bare ground of the SE cable. From there they used one insulated leg of the SE and the bare ground to make the circuit in the breaker panel. The other insulated conductor of the SE is capped off in the breaker panel.
At least they used a 20A single pole breaker.
They also didn't bond the JB.
That's what I'm going to do. What you do is up to you (and AHJ).
steve