Steps:
Navigate to the section you want to add the item to.
Tap on the large green action button on the section screen.
Tap on the "+/- from item catalog" button
This will bring up the full item catalog. Tap on any of the items that you want to add to the section, you can add as many as you'd like from the catalog. Note: the items currently selected will be highlighted in green, you can also tap on any of them if you want to remove them from the section.
This will create an alias of an existing menu item so that it can be re-used in another section. You can change data points such as the name, images, and/or description for this aliased item, but if you make a price change, it will update all of the other instances of that item. E.g. if you add an existing "Hamburger" item to another section, you can change the name to "Monthly Special Hamburger", and if you edit the price, it will change the price of both the "Monthly Special Hamburger" and the "Hamburger".