How do I run a new conductor on an existing kopex tube?
Without knowing the details for the Kopex tube (though it looks like it's probably some sort of flex conduit from a quick search) The normal way to &quot;add wires to conduit&quot; unless it's a very short distance is to shut off the power to the affected circuit(s); disconnect the wires at one end; attach a rope; pull the wires out the other end; pulling the rope in, attach the new wire, and pull them all back in together.
While this appears (to the uninitiated) to be a lot more bother than &quot;just adding a wire&quot; there is no &quot;just&quot; to adding a wire unless it's a very short, preferably straight section of conduit.
If there's no wire in the conduit in the first place, pulling a string in with a vacuum or getting a fish tape through are typical ways to get started. My personal experience favors the vacuum.
Using a fish tape or rod in a conduit with wires in place risks damaging the insulation on those wires and generally gets all tangled up in them anyway.