There is no use in adding flash files. If it could do it, DVD Arch would need to convert the flash file into an MPEG-2 video stream in order for the flash movie to be played on a DVD Player. If you happen to have macromedia Flash installed on your system, you can export the flash source file as a DV AVI, which would then be used in DVDA (or in Vegas).
Using something like DVDA 3, it would be possible to make a hybrid DVD and you could have the flash files included on the DVD along with your regular DVD video, and a computer that had a flash player installed could then play the flash files.