How would you handle the need for a grommet as required in 300.4[B],1 when adding an additional receptacle in a finished room, feeding horizontally from an existing receptacle into the next stud bay with metal studs using NM cable?

This is assuming that MC cable isn’t an option, since the finished room is already wired in NM and the boxes are one-gang nonmetallic.

TKX

Sleeve it in smurf tube. That is listed for direct threading through steel stud KOs without a grommet.
You should be able to thread the RX through a precut piece of smurf and push the assembly through.
If you want to be a fanatic, put female adapters with bushings on the ends but I doubt smurf will harm a RX jacket.

Gregs solution will work, but there is also a commercially available plastic grommet that pops right into a metal stud hole. They even have them at the orange box, as well as your local supply house.

Good Luck!

Greg's method sounds like the only way to go in 'fish' situations.

Thanks for the replies.
I had considered using 1/2" ENT, but the problem I find, is that if the wires to the existing receptacle outlet are coming vertically down the stud into the box, which is mounted on the opposite side of the metal stud, I can’t really fish up through and then back down the large factory punched hole. So, I usually either must find or drill a hole through the stud below the level of the box and then fish down through an empty KO in the bottom of the nonmetallic box to catch a second horizontal fish tape and bring it up through.
Another issue sometimes is that the Diversibits for sheet metal, only to go up to 3/4", so are not big enough the get 1/2" ENT through or even a snap-in bushing, assuming I'm able get my hand in there to install it. This basically restricts me to using a sleeve of 3/8" flexible metal conduit without fittings, which I always carry on the truck, but I’m not really this is kosher.
Luckily, I don’t run into this very often, but I think what is really needed, is a Diversibit that has a step-drill bit that can make a hole large enough for 1/2" ENT or possibly a standard 1/2" snap-in bushing.

I recently noticed that there is 3/8" ENT available. I suppose this would work, but then I would have to both stock and carry yet another limited use specialty item.

Kjay:
Delving into this deeper (for one outlet).....

MC can be used in a non-metalic box. I cannot locate any info as to IF the clamps in an old work NMbox are OK (acceptable) to secure MC to the box. The discussions went either way, with no documentation for yes or no.

MC in NMbox is NOT a 'normal' method here; I have seen it once or twice, but the MC was stapled per NEC.

I think MC cable would be ideal, but since the boxes I see are usually just the standard NM type with no clamps, I'm not sure it could comply with 314.3 or 330.40.
Changing out the box would likely be major hassle too, since they normally use either an MS or B-bracket, sometimes called a flag, that mounts on or over the face of the stud.

I would most likely have a hard time finding a suitable old-work box with cable clamps that's big enough to fill the hole left by a new-work NM box after removal anyway.
I suppose this lends itself to using a two-gang replacement box, but that may not be to the homeowner’s liking. I'm not sure if a Smart box would work in this instance either, since the remains of the original boxes bracket might interfere with the mounting of it.

It's seems that metal studs have been used in construction for quite a long time now and manufactures should be offering more listed products to deal with retrofit situations like these, not just new installation issues.
The only items I actually know of that are available for metal stud electrical retrofit work are just the same standard old-work boxes, fish tapes and a few inadequate sizes of Diversibits designed back in the mid 1980's.

My question would be how you bond the armor of MC in a plastic box? Unbonded armor creates the possibility of energizing the steel studs and that is a concern in Florida since we killed a guy that way.
I suppose if you can find an old work box with a 1/2" K/O you could use a MC connector and a bonding bushing (one end or both, up to the AHJ).

A bit extreme (sarcastic), You could rip part of the wall out and charge more. Perception is reality, works for the plumbers.

Greg:
That's a point that never occured to me!

Remove plastic box and install a deep metal old work box. us enmc clamp in the top and m/c clamp in the bottom. Madison bars to hold it in some patching my be required depending on how well drywallers cut there boxes in.

I have done this before. it works.

ob