Please see this github feature request and this forum post for details on why this isn’t likely going to be implemented (at least not anytime soon).
Given that you mention very sensitive data I think it’s worth emphasizing that adding a pin lock to Joplin would most likely just give you a false sense of security. Joplin stores everything un-encrypted on disk, that means that even if a pin prevented an attacker from using the application, they would still be able to read the notes directly from disk. The best solution for you is to follow @laurent’s advice and encrypt the application and it’s contents. This makes sure that your files are always secured up to the standard of whichever encryption software you choose.