Importing images from an older site to be displayed in Media

Hi Heroes

What is the best way to import images which have been stored in folders of an older Wordpress site and to display them all in Media?

Old site Uploads 2014, 2015, 2016, 2017 always with their subfolders 01 - 12
Now those should be moved to a new multisite installation which will use the same images but they don't display in Media. How can we see them in Media and Manage them?

Kind regards
Andi