Insulin can only be administered intravenously or subcutaneously. It cannot be administered orally. While I don't work in NNU, in my experience in similar environments (adult intensive care), any nurse can get the keys to fridge or medication cupboard by getting them from whoever is holding them (a frequent refrain is "keys please!" or "who's got the keys?!"). Syringes are usually easily available on a trolley close to the clinical areas. It would be very easy to get the keys to go to the fridge for another reason, and if noone is looking, slip a small vial of insulin in one's pocket. Similarly, it would be very easy for one to slip an insulin syringe in one's pocket. If one was so inclined and didn't want anyone to see one drawing up insulin it could even be done when one was in the bathroom (disgusting I know, but possible), then slip the drawn up syringe back in one's pocket.
In summary, IMO there would be opportunities for any nurse to covertly obtain, draw up and inject insulin into a TPN or dextrose bag if one was so inclined. All hypothetically and IMOO.
Mary could elaborate further!