When using PAM (the default), JupyterHub doesn’t have any mechanism for managing passwords, only checking them. It also won’t create users that don’t exist on the system unless you ask it to with c.LocalAuthenticator.create_system_users = True. If a user is created in this way, it will not have a password, but you can create one at any time with the standard system mechanisms, e.g. passwd USERNAME.
JupyterHub also doesn’t have a notion of an ‘admin’ user that’s not created by your explicit configuration (or your Authenticator). If you have users in the JupyterHub database and/or your configuration that don’t exist on the system, this should produce some warnings on startup, like:
KeyError: 'User some-user-name does not exist on the system. Set LocalAuthenticator.create_system_users=True to automatically create system users from jupyterhub users.'