The plugin is not able to do this. I was speaking with someone recently who is contemplating writing a plugin to convert the built in wp_list_pages() to do this but he’s very busy at the moment so may not get on to this for a very long time (ever?).
I don’t know if this option is satisfactory, but I just tried and it seems to do the trick; simply install the "Page Links To" addon – and target the top level menu item to the external page "#". It doesn’t seem to reload or anything, so I’m quite happy with this workaround so far.