A UL label means the item met the appropriate standards when it was built/shipped by the manufacturer. So, technically anything you do in the field, including punching conduit entrances can "violate" a UL Listing. UL would be the only one that can definitively say that a field modification has caused an item to no longer meet the standards under which it was tested. It is up to the AHJ to approve field modifications but UL will be glad to assist the AHJ in making its decision.
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.