Well, you will have to use the -o option to allow creating duplicate UIDs along with the -u, because you will have to create a user with the same UID as root.
for example suppose you want to create a user called user1 that has root privilages and that the original root user UID = 0, what you gonna do is as follows
useradd -o -u 0 user1
don't forget to add the root specific paths and environment variables to user1 as well.
hope this helps.
Rgrds.