How to make flat worlds start to generate like normal worlds in mcedit for mine craft?

Me and my friends have been working on a minecraft flat world in creative for almost a year now. I want to make it so that after a certain point, it starts to generate regular terrain, such as hills, mountains, oceans, snow areas, deserts, etc. How would i do this? Who ever answers the question best gets best answer.

Sorry I'm not sure if it is possible but since you are in creative you have unlimited sand and dirt you can make your own mountains and desserts how you want but there could be a way but I'm not sure sorry.