Sudoers Example: su - root

As root
#visudo
add below lines, where user123 is uid that need su - privillage
user123 ALL=(ALL) /usr/bin/su -
user124 ALL=(ALL) /usr/bin/su -
user125 ALL=(ALL) /usr/bin/su -
user126 ALL=(ALL) /usr/bin/su -
:wq!

as user123
user123% sudo su -
We trust you have received the usual lecture from the local System
Administrator. It usually boils down to these two things:

#1) Respect the privacy of others.
#2) Think before you type.

Password:

Authorized access only!

root#