There is a way to do that.

Scenario Data > Custom Win Condition > Custom Win Text

You can't change the ending unless you go with the custom win condition. However, it's still possible to create a message that will pop-up very NEAR the end of a given game. Just go to the events lab, create the ending title and text that you want and then select the appropriate variables, i.e. traits that need to be/not be evolved, population numbers like 99% dead, and especially 0 healthy people left worldwide because the last thing you want to do is have a custom win message pop up saying that everyone's dying and the player is about to win, only to get a game over because you didn't know that the Carribean wasn't infected or something.