lbartsch said:
Will adding kirk-key interlocks to existing panels violate NEC or UL?
The NEC does not much care about what you do inside a control panel. It just does not cover the inards of control panels, other than a very few things.
Generally, if you install a UL listed part in accordance with the manufacturer's instructions, UL is happy.
The people who original built the control panels may have an issue with you modifying them and expecting it not to affect the warranty.
The other thing to consider is that normally this kind of thing is added to provide some additional level of safety. You may find that you are taking a lot of liability on without increasing the safety of the system by a whole lot.