Scrivener is somewhat limited in this regard, in that it is limited to using OPML. Have you tried using the built-in template OPML? This can be added to your TBX using the File menu.
The template assumes you want to export a note and all its descendants, be that one part of the outline or the whole document.
With the template added, to export just a branch (i.e. container and below), select the container. Open the HTML Inspector, Export tab and select the ‘Scrivener’ template. Then use File menu → Export Selected note. This will create an OPML file that Scrivener should be able to ingest.
Note that even if your source note $Text use RTF feature, OPML text is limited to plain text. That is a limitation of the format choice and the format choice is a limitation of Scrivener. So if you want rich text support you need to go lobby at the Scrivener end of things as that will need supporting a format other than OPML.
Does that help?