To add new files to a CD-rw or DVD-RW disk you have to reformat the disk and reburn the entire thing with the new files --- Which means erasing all of the contents that are on the disk so is not really the format you want -- what you are wanting is DVD-RAM which is a format similar to a HDD or USB drive where you can reuse the disk much like a HDD and write files to them and erase\add more files without rewritting the entire DVD.