Hi fgavioli,
you can add the photos you have just as a external library https://your-immich-URL/admin/library-management
Than you have the photos in immich after a first scan, that can take very long, if you have a large collection off photos.
Hope this helps
Just read through that feature discussion. That would be idea for my needs as well, eventually I will want to go through and cleanup those 20+ years of photos, remove duplicates, etc. I’d suggest folks reading this thread go give that feature an up-doot on GitHub so the Immich authors know how much demand there is.