I don’t see any way to do it in Wavelab 6 either. But you can do it with “customize commands” in Wavelab 8. There’s still no default shortcut key(s) in the menu for paste multiple copies, but the command does appear in customize commands so you can create a shortcut there. Maybe try the Wavelab 8 Elements demo to try it?
Or you could try to find a third party menu shortcut program, but I haven’t found an easy one since Quickeys for Windows stopped developing, although the version they still sell might work for you. Haven’t tried AutoHotKey yet, but I’ve heard there’s possibly a steep learning curve with that.