Can you add a cable connector in knockouts in a breaker box after the wires are already in place?
Yes, there are connectors that can be slipped onto the cable and popped into place in the panel knockout. They do not snap over the cable though, you must disconnect the wires and slide the connector over them.



Also, here: http://www.amazon.com/Hubbell-4714B5-Connector-Thermoplastic-Installs/dp/B007IAMVFW/ref=sr_1_19?s=lamps-light&amp;ie=UTF8&amp;qid=1417826273&amp;sr=1-19