It would be great if lightroom classic could import/copy photos to one central location and leave the original where it is. this is what mac os photos does. in this manner, one can make a backup of all files from one place. also, one can delete the photo files that may be scattered around the computer.
Huh? Maybe I misunderstand your suggestion, but Lightroom can already do that since day one. Just select 'COPY' at the top center of the import dialog. The only problem is that this won't give you a backup if you import from a memory card (that you erase and reuse after you imported the images).
No, don't use ADD. That is probably what the OP does right now. The OP wants to copy files (that are now scattered all over the drive) to a central location, and wants to delete those scattered files later. That means he should use COPY. Alternatively he could use MOVE, so he does not have to delete any copies later.