The nuclear codes weren’t technically connected to the nuclear weapons - they didn’t “unlock” anything. They were just an authenticator sent to the officers in charge of the control station. Only the US implemented a remote-unlock system for some weapons, toward the end of the Cold War.
“Technical expertise” needed would have been minimal. All of these weapons could have been dismantled and the nuclear material (plutonium) removed at the very least. That is the main requirement for building a simple nuke. And there probably were Ukrainians who could have maintained much of the weapons materiel left in Ukraine at the time.
Of course they could...which is precisely why the Russians never let Ukraine have control of them. At the time corrupt Ukraine officials were selling every piece of Soviet military hardware that wasn't nailed down to the highest bidder.
Even after the nukes were transferred out of Ukraine their President whined to the Russians about not 'getting paid' compensation for the warheads.
Ukraine has always been a cesspool of corruption run by worst sort of criminals.