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']"
https://askubuntu.com/questions/332264/13-04-more-than-four-workspace-shortcuts-in-gnome-flashback-no-effects