There are now helper links that allow you to add a case to the top/bottom of the backlog in edit mode. You can also set/change a case's backlog value when viewing a case (i.e. not in edit mode).
These improvements will go out with the an updated version of the Project Backlog plugin (i.e. within version newer than v1.1.4) which should go out in the next week or two)few weeks.