At the moment you can not have separate passwords for your pages. There is however a workaround.
The first step is to create a website for each page you are wanting to have a separate password for. You then create your main site and you set up the menu links to each page that you want a different password to as separate pages. Then link your menu links to the websites that you created earlier by creating page redirects.
For information on redirecting a page automatically, please take a look at this link: Links that redirect when you click on the menu tab.