Use Data Import/Export | Scan for Keywords to use the keywords that are already in your pictures as raw material for building or supplementing the keyword list contained "inside" Zoner Photo Studio.
As relates to actual individual pictures, Zoner Photo Studio does not maintain their keywords in a database of its own; it stores keywords directly inside image files, in their EXIF data. This way, pictures will not lose their keywords no matter where you move them, and any EXIF-aware program can process their keywords.
We recommend that you use this function when, for example, upgrading from previous versions of Zoner Photo Studio. This way, Zoner Photo Studio will have a complete list of the keywords you have used in the past, and will offer them in all of its windows where you can search keywords or add them.