Adding custom page properties is not supported in Sitefinity 3.x. We will implement this from Sitefinity 4.0 where the page API will be completely different. Your request is not clear about whether you what to implement this functionality - backend or front end.
1) not show in sitemap
You can access the RadTreeView control that shows the SiteMap and get access to NodeBound event. Then hide the nodes you do not want to see. The control is located under ~/Sitefinity/Admin/ControlTemplates/Pages/SiteMapPanel.ascx
2) if someone browses to the page, it will not show the page, or any of its history,
It will simply show a 404 page not found
After you are done with Step one the page will not be bound and it will not be accessible.
the Telerik team