A relatively new feature is that most nodetypes can do something with a url attribute.
For example, if you have a blogpost node with a url, then the OPML for the post is gotten from that URL and appears as if it were subordinate to the blogpost node.
That's the repeating theme. So an outline node or a thread node can specify a URL.
What's missing is an easy way, in the user interface, to set the url attribute directly.
1. Right-click on the node.
2. Choose Outline from the Types sub-menu of the World Outline sub-menu. Confirm.
3. Choose Set URL from the right-click menu.
4. Enter the URL of my states outline.