Perhaps I have misunderstood there being a second component to the question regarding export. My first response was meant to address importing stylesheets—we don’t, yet, but we will in time and then all of this will be easier.
This probably the most comprehensive thread on importing a stylesheet-based outline. If plain-text is a fine alternative, you should be okay with the methods in that thread.
For formatted text, that is going to be the best solution, even though it only has one “level” of depth.