If they are original dreamcast games then you can not make a backup very easy. With backup games that were burnt on a cd-recordable you can just make an image with your favourite burn program or do it with padus discjuggler to get cdi images. And yes you can make cdi images with exoboot only if they are backup dreamcast games too. Copy all gamedata to the data folder where you installed exoboot and use the 'data to cd' method to make a selfbootable cdi image that only can be used with Daemon Tools 3.29.
But if they were backup games you could also play them directly with your cd/dvd drive in Chankast. These Dreamcast discs are original, right? If this is true then the procedure is somewhat complicated and you'll probably just have to post again when you get all of the equipment to make your own backups.