To add a category header, simply navigate to your inventory by entering your created stream, and tapping the shop icon. Then select the + icon on the bottom right.
Once in the item addition screen, tap the "Category Title" slider. This will allow you to add a category to your inventory.
Created category headers will not appear until an item is added and saved within that category. If a category header is added before items are in the category, the header will not appear, however once an item is added to that category, the category will appear.