Importing images that already exist on your server is exactly the same as importing images from a URL, except you only need to specify the file name of the image.

The images you are importing must be uploaded to /wp-content/uploads/wpallimport/files/

The images in this folder will be copied to the paths used by your WordPress Media Library when the import is run – typically, /wp-content/uploads/YYYY/MM/.

When importing images like this along with the Scan through post content and import images wrapped in <img> tags option, the image URLs in the content must be absolute such as https://yoursite.com/yourimage.png and not /yourimage.png.