O que você está tentando fazer é impossível sem baixar a fonte e programá-la em si mesmo.
Veja:
gsettings list-recursively | grep-red --ignore-case "switch" | grep-blue --ignore-case workspace
org.gnome.desktop.wm.keybindings switch-to-workspace-down ['<Control><Alt>Down']
org.gnome.desktop.wm.keybindings switch-to-workspace-1 ['<Super>Home']
org.gnome.desktop.wm.keybindings switch-to-workspace-2 ['disabled']
org.gnome.desktop.wm.keybindings switch-to-workspace-3 ['disabled']
org.gnome.desktop.wm.keybindings switch-to-workspace-4 ['disabled']
org.gnome.desktop.wm.keybindings switch-to-workspace-7 ['disabled']
org.gnome.desktop.wm.keybindings switch-to-workspace-5 ['disabled']
org.gnome.desktop.wm.keybindings switch-to-workspace-8 ['disabled']
org.gnome.desktop.wm.keybindings switch-to-workspace-9 ['disabled']
org.gnome.desktop.wm.keybindings switch-to-workspace-6 ['disabled']
org.gnome.desktop.wm.keybindings switch-to-workspace-10 ['disabled']
org.gnome.desktop.wm.keybindings switch-to-workspace-11 ['disabled']
org.gnome.desktop.wm.keybindings switch-to-workspace-12 ['disabled']
org.gnome.desktop.wm.keybindings switch-to-workspace-left ['<Control><Alt>Left']
org.gnome.desktop.wm.keybindings switch-to-workspace-last ['<Super>End']
org.gnome.desktop.wm.keybindings switch-to-workspace-up ['<Control><Alt>Up']
org.gnome.desktop.wm.keybindings switch-to-workspace-right ['<Control><Alt>Right']
Nenhuma configuração para alterar esse comportamento ...