You'll have to modify the fr file in /usr/share/X11/xkb/symbols.
Make a copy of the file somewhere.
Modify it as necessary.
Make a backup of the original file in the original folder.
Overwrite the file with the modified version (you'll need administrator rights:
gksu nautilus in terminal). BE CAREFUL. YOU COULD BREAK THINGS!
Restart X server or reboot.
P.S. If you're using Ubuntu 12.04, you'll have problems with the modified layout (I do) but there is a temporary workaround. In a terminal type:
sudo setxkbmap fr
I'm not using the standard french layout but the one called french alternative and french alternative latin 9 only. I have to use:
sudo setxkbmap "fr(oss)","fr(oss_latin9)"
Modify the command accordingly.