As we established in another thread, you basically can't use Python to make this happen. Its just one of those limitations - just like with XML modding. But, if you wanna encourage colonization, you could have the building spawn something like a explorer unit in a caravel at intervals. Or settlers for that matter. Not very intuitive, I know, but it would be doable. Or, the Civ in question could have a UP of spawning colonial era units - and the rate could be increased by building these buildings. So lets say that explorers and settlers report to the king in the capital at some interval. This interval would be affected by the number of buildings of this particular type. Makes sense?