Photos.app copies all the photos it manages into a separate repository, called "Photos Library".
On your Mac, go to your Pictures folder, and look for Photos Library.
Right click on it and choose 'Show Package Contents'. This will open up the repository into a folder view, showing lots of files, and a folder called "Masters". This folder contains all the unedited versions of your Photos in the Photos app. (because Photos is non-destructive, edits are stored outside of the image, just like Lightroom).So, "Masters" is where your photos are stored. However Lightroom can not access it.To allow it to access this we need to copy it to a different location.
Right-click on Masters Folder Copy it and paste it to your External drive or to your Desktop(if there is enough space)
Open Lightroom, and click 'Import'. Navigate to the location where you put the Masters Folder.
Simply Import the pictures clicking on Add.(You need to leave external drive connected)