Someone did this a long time ago (in a galaxy far, far away... oh wait; wrong universe; sorry
).
Anyway, they created a special colonization tech field, with only one component in it, which allowed colonization of any type of world (gas, rock, ice). They then made this colonization tech a racial trait and set the cost fairly high (to prevent player abuse, of course). I haven't played with this much myself; I've been trying to leave research areas, etc. alone. I did a fair amount of modding in SE3, but it was too easy to create seriously unbalanced items. So I move more cautiously now...
Anyway, hope this helps!