13.04 - more than four workspace shortcuts in GNOME Flashback (No effects)

I'm using GNOME Flashback (No effects) on Ubuntu 13.04 and I've got six workspaces however when entering System Settings -> Keyboard -> Shortcuts -> Navigation I only see options to address the initial four.

It used to be options up to 12 workspaces there and as I certainly don't need that much, I want to have at least enough for the amount currently active.

How can I restore or add more shotcuts specifically to navigate and move windows to workspace 5 and 6?

The only way I have found to modify the keyboard shortcuts for workspaces 5 and up in GNOME Flashback, is through a shell using the dconf command.

$ dconf read /org/gnome/desktop/wm/keybindings/switch-to-workspace-1
['<Primary>F1']

Here I read the shortcut setting for the first workspace, to get an idea about the syntax to use when setting values for the missing shortcuts. I'm using CTRL+ F1 myself.

To add shortcuts for the remaining workspaces, just modify the value returned above to match the workspace number, and use dconfto apply them:

$ dconf write /org/gnome/desktop/wm/keybindings/switch-to-workspace-5 "['<Primary>F5']"
$ dconf write /org/gnome/desktop/wm/keybindings/switch-to-workspace-6 "['<Primary>F6']"

Maybe a slightly more comfortable way then using dconf on the command line is dconf-editor. Run dconf-editor and then navigate to

org -> gnome -> desktop ->wm ->keybindings

There you can find and edit the switch-to-workspace and move-to-workspace commands for workspaces 1 through 9.