Hi @mortonprice, It doesn't look like you can add to an existing Minecraft world, according to the documentation. You should be able to read back the world you already created, and write that to a new world, together with the new geometries representing trees and buildings.

Got it working, As @XiaomengAtSafe said, you cant modify a world, but you can create a new one, for Example (World 3 a New world) contains (World 1_Old) and (World 2_New content) and combines the two together. However it wouldn't work for me unless i put each world level folder on my desktop. Kept saying i didn't have permission otherwise.

Keep in mind that some data could get lost. Last time I tried chests got invisible and bed colour changed to red.This is because block entity data is not written.