How do I create a administration page (with overlay) through the UI
You're probably looking for the Overlay Paths module:


  This is a simple module that enables site builders to use the built-in Overlay for non-admin pages.


However, if the path you've got for the node is under /admin I'm pretty sure that if you configure permissions properly you won't need that module; the overlay should pop up automatically. You'll need to give your users the "Use the administration pages and help" permission, as well as the one to actually use the overlay.

As it's a node, whatever node access solution you have in place will determine the access beyond those two permissions.