Not possible with the current beta. I plan to add a way to semantically mark different XHTML flows as one of the "fundamental structural components of the publication", as the OPF spec puts it for the <guide> element. It's not currently on the tracker, but it's on my TODO list. It's simple and easy to do, and should make it for 0.2.1, if not 0.2.0 itself.
Direct OPF and NCX editing is also a planned future feature.