I spent some time browsing through Roxio's support site - looking for information about multi-session DVD burning. Roxio says there is no standard for doing that, so they don't provide that support with their software (Toast) Seems that trying to leave a disk appendable is quite unreliable, and not supported by OS X, so even if you could append data, that data might not be readable, even on the same drive that writes the data.
Unless someone can prove me wrong, you can't append files to a burned DVD reliably, except for the limited use of DVD-RAM, which requires a drive that supports DVD-RAM.