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I believe this is hardcoded and so it can't be modded unfortunately.
Sorry. :(
 
Gytis said:
Maybe there is a way to mod colonisation chance then?
I believe it is decided at the time of sending the colonist - so you could edit the save to do it. However, beyond that I don't know.

EDIT: Talking of editing, you could edit the size of the colonies in the save also - making it a city if you so desired. I don't know whether you could make an event for that maybe?