The NPT does not permit any new signatories to opt-in to nuclear weapons status and you can't apply for a membership upgrade. That is why Pakistan and India didn't sign and North Korea left the treaty. The only way Israel can sign on is as one of the 185 declared non-nuclear members and all that implies. Adding Israel or any other country as a nuclear power requires a new treaty or amended treaty and there could never be a consensus for either.
U.S., allies conduct 22 strikes against ISIS targets in Iraq, Syria (Reuters)
from the article: Obama 'strongly' opposes singling out of Israel at nuclear conference