When copying some concert video DVDs, I used dvd-decryptor to copy the image to my HD, then write the image back out to the DVD burner.
I don't think that you need to take the file structure out of the image unless you plan to alter the content. If you want to alter the content (recode to lower datarate, extract parts, etc), then use dvd-decryptor to extract the file structure from the DVD to your HD. You can then do what you want with the content. Burning is then a matter of either creating an image ( I use imgtool) and writing that image to dvd, or using some program that can burn directly from the file structure.
edit: BTW, why use nero? If you just want to copy the image, dvd-decryptor will do it all.