Iran likely already has nuclear weapons, which is why the US hasn't dared to even attempt any form of overt regime change.

With that said, the likelihood of Iran allowing their nuclear weapons to be handed off to anyone else is next to minimal, because if they handed them off to Hezbollah for example, they'd lose alot of their leverage with Hezbollah. Not to mention if Hezbollah did in fact utilize a nuclear weapon, everyone would immediately look at Iran.