You can do that in Sigil but in PageEdit since we do not "manage shortcuts" any assignment of a new shortcut would have to be permanent and work cross-platform.
I will search the PageEdit code base to see if your proposed Ctrl+Shift+N is used anyplace. That said a shortcut for something that can be done with a single toolbar click, that is typically only used one or twice if at all in an epub and that can be done in Sigil before launching PageEdit is low priority.
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