i don't know about k3b, but burning a cd from an iso is really easy from the command line:
cdrecord dev=0,0,0 speed=16 -pad -v example-iso-image.iso
just be root, and replace the speed for whatever your speed is, and the "dev" to wherever scanbus picks up your burner...
cdrecord -scanbus
http://www.troubleshooters.com/linux/coasterless.htm
maybe at the k3b site you can get some info about it:
http://www.k3b.org/