Converting between OPML and markdown makes bullets equivalent to headers (with the h1-h6 hierarchy) and lines below the bullet into text. So there is a natural way of doing this, so long as you maintain awareness of the difference between paragraphs/bullets and lines.
My problem was with the shortcuts. I wanted to make the behaviour the same in document and outline mode, but changing shortcuts always gave the inverse action in the other mode. I found this very irritating because toggling document mode accurately switches bullets on in paragraphs, and not lines, but the keyboard behaviour was different. Now solved by editing config.edn as advised above.