Is it possible to change the default GNOME/Ubuntu lock screen function (the Lock icon in the Panel's System Menu)?

I want to change the default lock screen application to i3lock. I can invoke the i3lock application through the terminal easily enough. But I would like to be able to use the default GNOME/Ubuntu UI and select the lock icon from the System Menu (I think it's called that -- see the image below). There is no option that I could find using dconf (I am using X11). Is it possible to change the default GNOME/Ubuntu lock screen function used by the Lock icon in the Panel's System Menu?