Also, keep in mind, most Chapters are not individual files, they are only bookmarks that allow the DVD players to fast forward. Now, if you have a menu movie that plays at the selection screen, that is considered a chapter, and is generally a different file, same goes for "extras". But what you are really dealing with there is moving from iMovie to iDVD to create the DVD application that finally gets burned.
I would copy the video selection you wish to burn to DVD and put it in a separate project and burn that project.