I've old backup from my MacBook (2008-2010) iPhoto library on external HD, i wish to browse + select photos and merge them with my current iMac Photos app in El Capitan 10.11
I had the same problem. You've got to do:
- Open PHOTOS while pressing OPTION key. Choose form a list your old iPhoto library. Either Photos will convert the old file or will direct you to a link on Apple where you can download an utility that will convert the file for you.
- In any case you will have to repeat and open Photos with option key pressed. Choose the converted file.
- Once it is opened you have to select all photos and EXPORT them to a folder in your desktop.
- Then you go through your filtering. After having all the photos you want in a folder, reopen Photos with the option key pressed. But now choose your regular Photo Library.
- Then you IMPORT the photos from the folder.
- In case you need the convertion utility from Apple, here is the link: https://support.apple.com/pt-br/HT202358 . (It did not work for me. I think El capitan doesn't like these old utilities - from 2013.)
Hope you succeed.