Fishing extra wire through old Elec box, that has wires already
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.