In addition to copying photos from another Photos library, you can also import new photos directly into one of your Photos libraries by dragging the photos from the Finder and dropping them onto a library in the PowerPhotos window. PowerPhotos will automatically open up the library they were dropped onto and import the photos in Photos for you.
Unlike when copying directly from another library, there will be no additional metadata such as ratings, keywords, etc. for PowerPhotos to copy across, though Photos will still recognize information embedded in the photos such as the date it was taken. This does still have the advantage of being more convenient than opening the library manually, plus you can still take advantage of PowerPhotos’ duplicate detection system when importing photos this way.
Additionally, if the folder you drop has your photos organized into a hierarchy of subfolders, PowerPhotos will recreate that same folder hierarchy as albums organized into folders in your photo library.