No they couldn’t otherwise people could hijack nukes regularly. This is why the U.S. codes are on floppy disk still there are a number of safe guards put in place to prevent such things from occurring. This isn’t splinter cell, James Bond, or 24.
1. If you have PHYSICAL ACCESS to a nuke. You can take the mechanical warhead and configure it however you want.
This isn’t a widespread problem because countries don’t allow access to their nukes. You have no idea what you’re talking about. Do you think there is some special magic that protects the refined plutonium/uranium from being taken apart? Do you think there is nothing engineers can do when they have control of a nuclear arsenal?
2. Ukraine had physical access and complete control over a nuclear system. The only thing stopping them from reprogramming launch vehicles/detonation control was assurances that they would be protected if they gave them up.
3. What career field do you work in?