You can export via the tree the existing structure, you will get something like:
. = space, Ive used . to show where a blank space would/should appear
.Top ..Child1 ...ChildLevelThree ..Child2
Then you need to create a new structure, you can simply add more spaces to the beginning of the lines to indent as necessary for adding into the new structure.
.NewStructure ..SomeTopic ...SomeChapter ....Top .....Child1 ......ChildLevelThree .....Child2 ...AnotherChapter ..OtherTopics .etc
Hope that helps