I’m not sure I understand your meaning. Do you want to add your existing backups to the backups folder of your new installation? If so, just copy the backups to the folder pointed to under Tools->Options->Backup.
If you want to take a backup and “restore” it so that it becomes your working copy, then just copy the most recent backup to your documents folder (or wherever you keep your non-backup projects). If the filename ends in .zip, then double-click it and choose to extract all contents. Be sure to rename the project folder (the .scriv folder) if it has a date stamp on it, unless you want to keep that.