Japan would need to amend their constitution for that, and that is simply not happening. They have failed to amend their constitution for much more mundane objectives, let alone nukes.
As it is, Japan probably already has nukes that are all-but-assembled. Even if they don’t, they’re at most a handful of months away from taking whatever steps remain for a highly advanced industrial economy with existing nuclear infrastructure to finish building a bomb or two.
Remember, Sweden and Finland were staunchly against NATO membership until Russia invaded Ukraine.
Constitutions are not supposed to be absolute constraints. They are not supposed to prevent a country from doing what is reasonable or necessary, as long as people are willing to accept that it was reasonable and necessary.