I'm fishing 14/2 wires through some electrical boxes (outlets, and switch boxes), that all ready has wires in it. I'm not hooking it up to the electric, I am just fishing the wire, and waiting on the electrician to hook up elec.I never fished before. I did check housing codes, and what wire to use. They told me to keep old wires, and fish the new. Do I punch another hole in the old box to add more wires in it? Any other tips?
To just answer your question, yes you can punch out a knockout to add new cable to a metal junction box. You will need a connector similar to the one below to secure the cable in the box. There are certain limits on how many wires you can run into a box so you'd have to check on that with whoever is assisting you in this. If you're running wire into a plastic box, there are tabs you can bend in to insert the wires, no additional connectors are required.