Photos Import from Photos folder

You'd need the paid version of PowerPhotos.

Comparison of PowerPhotos vs Photos re Importing Metadata from OneLibrary into Another Library

I ran a number of tests with a 3119 photo, 72 video and 97 keyword Photos library with keywords, captions, keywords and locations.  I merged it into an empty library using Photos and PowerPhotos.  This is what I found was imported by each;

PowerPhotos:           Photos:

Albums***                                          -----------

Smart Albums (as regular albums)** -----------

Folders Containing Albums -----------

Captions                                             Captions

Titles                                                   Titles

Keywords* Keywords (some but not all: …

…see Note *)

Original images                                  Original images

Edited images                                     Edited images

---------------                                        Locations

Favorites                                             Favorites

Can detect and exclude                      Duplicates

duplicates upon import or not

Neither method could import/merge projects. 

Both methods could import older iPhoto libraries into a Photos library when the iPhoto library couldn't be migrated into a Photos library. 

Notes:

*The Photos app only imported 84 keywords our of 97. PowerPhotos merged 147 keywords. Photos only imported keywords that were embedded in the original file and not those added by Photos. PowerPhotos imported all of them.

**The original library had 90 Smart Albums. All were merged by PowerPhotos but, as indicated above, the Smart albums were brought over as regular albums. No album of any kind were imported by Photos. Also only PowerPhotos imported folders.

The original library had 34 regular albums and 1 folder with some nested albums. PowerPhotos merger all of them successfully.

The original library had 72 videos. Photos imported only 57. PowerPhoto got all 72 in its merge.

First, which Library is your System Library? You can see your Libraries listed by closing Photos and option-clicking on the Photos icon to see the Library Picker:

And it shows the path to each Library and identifies the System Library.

Are you saying that, in your System Library, Photos' "File>Import," when pointed at the other Library, says it can't import? "unrecognizable format" is what you would see importing a corrupt file, but I've not seen it with a Library package. Does the Library have a ".photoslibrary" extension? "File>Import" should say "Review for Import."

Best way is to use Power Photos to merge the two libraries:

https://www.fatcatsoftware.com/powerphotos/

Questions still with no answer:

First, which Library is your System Library?

Can you describe what actually happens when you use Photos' "File>Import?"

Photos Library 2 seems to not have an extension. What happens if you change it to

Photos Library 2.photoslibrary