I don’t have enough hard drive space and may delete files, but I just want to add files to backup without deleting them. What should I do
Are you asking how to backup files, and then delete them, with the ability to restore them later?
Then yes, any backup software should be able to do that. Duplicacy certainly does.
However you would need to restore revision created before your deleted files. Because in all subsequent revisions those files will be correctly not present.
Using backup to offload old data when running out of space may not be the best approach though.
Something like virtual filesystem with rclone that fetch files from the cloud on-demand may suit your usecase better.
Still, you do need to have a backup anyway, but in this case all of your files will be present on every revision, even though they don’t need to be present locally taking space unless you actively using.