It's easy enough to mod that change in- I did it myself.
I'm doing this off the top of my head (don't have my civ files on me), so pardon the ambiguity in some areas. I'll try to post a more detailed explanation tomorrow:
Go to the Stabillity.py file, and at the top somewhere it defines a list of colonizational civs. Add in whatever civs you want.
I personally added in Germany, Italy (I play RFC Dawn of Civilization which has Rome reborn as Italy), Russia, and Japan.