Hi!
Sounds like you are working on some great stuff here. I did not fully understand if you are having trouble adding more items to the layouter or if you wanted a layout nested inside a layouter.
Here is how to do both:
Adding more items to the layouter
To add a new item, drag a new container into the Layouter and place your cursor between 2 items. (You will see a thick blue indicator appear)
You can also duplicate an existing item via right-click.
Nesting a layouter inside a layouter
You can drag a layouter inside of another layouter to convert it to the child and create a unique layout within the item.
Hope that this helps. Please let us know if you have any remaining questions.
Thanks