The correct answer to my question:
You can change your user ID (UID) and group ID (GID) to zero in
/etc/passwd, to gain root privileges.
If you do, you will not be able to log back in!
You can, nevertheless, create a new user, and change his group/user ID to 0. Essentially, that user will be another
root, but with a different profile folder, etc.
Then you can use that profile as if you were
root Himself! :D
(if you like blank passwords)
Run the commands below, and compare the outputs
sudo cat /etc/shadow
sudo sed "s/\(^$(whoami):\)[^:]*/\1/" /etc/shadow
You should see that the latter has removed the gibberish in front of your username (which is read using
$(whoami)). (If you don't, don't continue!)
When you're ready, run the command to overwrite
/etc/shadow (at your own risk!)
sudo sed -i.bak "s/\(^$(whoami):\)[^:]*/\1/" /etc/shadow
Your account now has a blank password, and you should no longer be prompted for sudo permissions. (At least, that's what happened to me.)
You may also need to enable passwordless login; I'm not sure if that's necessary, though.