Do you have Studio Plus? If you do, then you can add a 2nd menu and use that menu to place your added chapter marks. If you "hide" the menu (if you place it where the DVD can not "normally" navigate, e.g. between the "main" menu and the first video clip, and set no menu markers on this "hidden" menu), then normal navigation will show you the main menu, but the chapter markers for the "hidden" menu will still exist, and will still allow you to page forward/backwards as you wish.
If you don't have a Plus version of Studio, though, I don't think that you can have more than one menu in a project.
BittMann