with Toast6 you can "master" a video-dvd as an .img and add on this the other files; then you can burn this img on a dvd-r. what's important: the VIDEO_TS and the AUDIO_TS have to be on the first level of the dvd.
with iDVD - afaik - you cannot burn "virtual" dvds.
so, a long way would be for me: author your DVD with iDVD (to get all the nice menus) and burn it (if you use a dvd-rw, you can use it later for other datas) ; then import from that dvd-r everything into Toast, add the other files and burn it again as DVD-Rom