Thats, not possible, I'm afraid. Basically, the idea is to use yWriter for outlining. As far as I know, you can import outlines made with other applications to create new projects, but not to extend existing ones.
However, if you have put more work into your outlines than into your new yWriter project, you can discard the yWriter project and create it anew using your complete outline.
By the way, what's the point of incrementally creating your outline outside of ywriter? Are you preferring a text editor on a mobile device?
Cheers,
Peter