photos.app – How to move some photos to external hard drive

I want to move some of my older photos from the library to an external hard drive as my disk is getting full. I do not want to export, delete and re-import the photos to keep face/keyword information. I also do not want to move the full library to the external hard drive as I like to have my more recent pictures stored locally.

Note that I still want to keep the pictures within the Photos library. I just want to move their physical location.