I was changing an outlet in my house yesterday and found out that the romex coming into the box had no clamp. Is it possible to add a clamp from the inside of the box, without removing the wiring? It's an older box with no place for a built in clamp. I'd need to add a round one.

I have not seen a cable clamp that can be installed without disconnecting the wires. Maybe we will hear from others who have.

Sounds like a wall case in a finished wall with a Romex entering a 1/2" knockout with no connector. Like that's never been done before.

I suggest just using a bushing type connector similar to :

Black Button NM Connector Now In 1"


Disconnect wiring from device, slide bushing onto wires and snap into knockout, re-install device.