Sure -- there are a couple ways. First way is to add the file to the DVD-ROM section of the DVD -- this allows the user (with a computer) to look at the file structure of a video DVD, open the DVD-ROM folder and use the files contained inside (copy or play them). Those files can be any computer format (Word, pdf, jpg, mov, etc).
If you want the knowledgeable DVD user to be able to access hidden files using the DVD menu system, the way I trick iDVD is to "re-name" the video so the video's name is a blank space. More precisely, drag and drop your video onto the iDVD menu. Change the iDVD menu setting so you can "Free Position" the menu buttons (this allows you to drag the special menu button to an appropriate hiding place). Highlight the name of the special video and replace it with a click of the spacebar. If you pass your computer cursor over this transparent/blank button, it will highlight and can be clicked. With the DVD player remote control, you can click down your list of visible menu items, and if you added the hidden video last to that menu, it will highlight after you click past the last visible item.
You can add a hidden submenu using the same concept -- add a new submenu, but change the name to a blank space (use Free Positioning to hide the menu button in plain sight). When the user clicks this hidden button, they are taken to a submenu where the menu items are normally displayed, or further hidden.
Can be fun. Works better with some iDVD themes than others.