mmm it seems strange the cd-rw behaviour you describe.. if i well understood you want to use it like a banal finder folder, where you put the files when you want... only dvds ram have this capability... anyway you can "simulate" (!) this feature by using ISO9660 multisessions: you have to burn your dvd using ISO9660 option without finalizing it and when you want to add some files, you have to import last session from "session" menu in ISO9660 window that appears when you click on "select" button in the main window...
and you can do this operations also with not rewritable dvds