... how to add root as a su user ...
Root already has all the power of the
substitute
user -- no need to change anything on your system. For updating your system, the recommendation has always been to do as sunrat has written. Or, if you are booting your system with a plan to update it, when the grub screen appears. press "e" and then add "3" at the end of the linux boot line and then Ctrl-X to continue booting directly to tty1, and login as root.
I've personally never used sudo in ten years of running a Debian sid system, nor smxi -- they are unnecessary and no one here will be interested in solving problems related to those methods.