this is a partial duplicate of the question Linux 'compose' key sequence extensions:
I am using ubuntu 13.04 with unity and I would like to have a specific composition sequence to work as defined in the X11 rules (<Multi_key> <acute> <c> produces now "ç") and to add a new one (<Multi_key> <z> <z> to produce "ʒ")
my ~/.XCompose contains both above rules, but the above behaviour is not here yet.