Create own dead_key

I am currently working to create a specific greek keyboard layout adapted to French Swiss Keyboard, I create a keyboard layout to be put to /usr/share/X11/xkb/symbols/( https://github.com/grecromand/grecromand/blob/master/linux/grecromand) . I used code `dead_grave. But I would like to create my own dead_xx. It would be dead_sigma which would produce a ς if followed by space or ponctuation mark, and σ` otherwise. I Have understood that dead_key are defined in /usr/share/X11/locale but not more. I have so many question: - WHich file should I modify? Is it possible to create this dead_key without modifiying distributed files, to avoir problem when upgrading my system? - How can I declare this dead_key?