Sure there is. Though I'm not sure how it works with K3b which is a graphical front-end to a variety of command-line programs like
cdrecord or
growisofs or whatever it uses, as cron is command-line and doesn't move your mouse for you. Cron can do anything you can, as long as you can do it from the command line
I would suggest looking at the command-line DVD recording tools, they're probably easier to use for a task like this. You can use
find in the cron command to locate the newest files (say files that are one week old or newer), create a list out of them and use that list along with a burning program to burn the files to a DVD.
Read man pages, or if you don't know exactly what you're looking for,