Im digging into the new procedural vegetation editor in the 5.7 preview. And it seems you cannot “create” you own tree, and are limited to editing the existing examples in the scene. in the data asset there is a refence to a .JSON file that seems to be a “recipe” im assuming that holds alot of the info on the type of tree. For example, you cannot make a pine tree as the existing trees are of the maple variant.
Im eager to hear some other view points on this topic and discuss it furhter