Adding more galleries to an existing folder requires only 2 moves.
1. Go to your Admin Panel -> Website -> Pages.
2. Drag and drop the needed gallery from the list of pages to the folder.
To learn more about drag-and-drop operations, please, click here. (Drag and drop operations on the menu)
If you would like to know how to create a gallery page, please, click here. (How to create a gallery page)