It would appear that it can't be done. I tried various work arounds - trying to copy and paste the bookmarks into other formats/programs - no go. They copy and paste, but turning them into something that the other programs recognize as a bookmarks list is another matter. The plist file can be opened in BBEdit, but what the heck you're supposed to do with it, is anyone's guess.
The only real workaround is to use URL Manager Pro - which can, apparently, import bookmarks from Safari.