Hi Nils,
In version 0.3, you can use Cut (Ctrl+X) and Paste (Ctrl+V) or Paste
As Subissue (Ctrl+Shift+V) while you are viewing a single page with a
structure.
In the upcoming version 0.4, the whole thing will be a bit different,
and, hopefully, simpler. (Cut-Paste will work in 0.4 anyway, but the
"clipboard" will be persisted when you move to another page.)
Hope this helps!
Igor