I firstly installed ChromeOS with Brunch and then Kubuntu on my Laptop. Now grub doesn't recognize Chrome OS. How to add Chrome OS to Grub?

So long as ChromeOS was properly working before installing Kubuntu, you can add a record for the OS in /etc/grub.d/40_custom that looks like this:

menuentry "Chrome OS" {
 insmod part_gpt
 insmod ext2
 set root=(hd0,gpt3)
 linux /boot/vmlinuz root=/dev/sda3 init=/sbin/init rootwait rw noresume console=tty2 i915.modeset=1 loglevel=1 quiet noinitrd tpm_tis.force=1
}

_{Be sure to set gpt3 and sda3 to the correct partition number if I've incorrectly identified the boot partition for ChromeOS.}

Note that the above configuration is assuming that your storage device is a GPT device. If it's not, you will need to remove the insmod part_gpt line from the configuration and change the root to (hd0,X) where X is ChromeOS's root partition number ... probably 3.

Once done, update Grub:

sudo update-grub2

Now you can reboot and find a record for "Chrome OS" in your boot list.

_{Note: If ChromeOS was not working before installing Kubuntu, you will need to confirm the STATE_DEV=${ROOTDEV_TYPE}1 setting in the /sbin/chromeos_startup on ChromeOS is changed to ${ROOTDEV_TYPE}3 (or whatever the root partition for ChromeOS happens to be). Only numbers 1 through 8 are valid, so you may want to be careful given the number of partitions on your storage device.}