Adding social logins to email/password accounts (or adding add’l social logins and/or email/password to accounts that don’t already use them) was recently discussed here:
See Gaby’s response, which points to the documentation for this. (In short, when a user is logged in via whatever method they use, if you present them an interface to login via another technique, that login method will be added as a way they can log in. In the case of adding an email/password combination to a user who currently logs in with an Oauth social login, it’s a little different – you execute the “Reset Password” action on them and they’ll get sent a password reset email which allows them to create a new password. At that point, they now also have an email/password as a way to login.)