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

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?