Right now it seems impossible to set up more than 4 keyboard layouts in gnome, at least in it's own preferences.
But you could workaround this by setting the keyboard layout directly, with a custom GUI, which makes use of "gconftool-2". I wrote a script for such a GUI for different purpose, you may just modify it:
The gconf-path is: /desktop/gnome/peripherals/keyboard/kbd/layouts
To lookup the values of your currently set up layouts, enter in a terminal:
or press Alt+F2, enter "gconf-editor", and browse down the path.
gconftool-2 --get /desktop/gnome/peripherals/keyboard/kbd/layouts
The actual command to set the layout would be like this:
You will notice that your currently set value contains more than one entry. This only works in combination with Gnome's build-in preferences. If the value contains only one entry, those gets used by Gnome automatically, I've tested it.
gconftool-2 --set --type list --list-type string /desktop/gnome/peripherals/keyboard/kbd/layouts [us]
Feel free to ask, if you need further help to implement it!