Superadmin is the HIGHEST group in Gmod.
Do NOT restrict it this way.
Instead, do not give out superadmin to those who do not deserve the 100% trust of the server.
Once a person has superadmin, they can cause much damage to the server WITHOUT needing to run a single ulx command.
I, no, we as team Ulysses just can't ethically support what you're asking.
Yes, there are several ways to do it, almost all require special command lines or editing skills of groups and or text files.
I believe from your lack of understanding how to accomplish it, shows me you're not ready to master this skill.