With Build It,
You cannot transfer/copy an object to another World.
If you want to use the object that you built, why don't you try updating the World that you created?
Please keep in mind that if you submit an updated World for review and it is approved, it will override your existing World.
However, you can transfer/copy your World created in Build It to Studio World.
Load the World you created in Build It to Unity editor and use it to create a World.