You can create password protected pages and you can link to them in your custom emnu. Doing so will result in a visible tab in your horizontal navigation but only those with the password will be able to access the content on the page.
Protected – The page is protected with a password you set. Only people who have the password can view a protected page.
It will still be a page with the characteristics of a static pages. It will not be a a specific part of the blog. You will only be able to update it manually.
The differences between pages and posts
Staff have provided this support documentation entry for those who want to structure their blog like a website. > http://en.support.wordpress.com/using-wordpress-to-create-a-website/ We organize our posts by use of categories. There is only one dynamic page in a blog for posts and we cannot post to more than that one page. However, we can create the appearance that we have done that by setting up a custom menu and displaying Categories in tabs along the horizontal navigation menu. Instructions > custom menus > http://en.support.wordpress.com/menus/
Read also> http://wpbtips.wordpress.com/2010/06/30/posts-on-pages/