On Debian or Ubuntu systems, useradd is a command itself, and you can create users and define options to them using this command, and adduser is a perl script, that uses useradd to create the account, asking you the password, Full-name, phone and others ..
On Fedora or CentOS systems, adduser is just a symbolic link to useradd, but may try to copy the script from a Debian system to the CentOS one, I have never tried it, and may need to modify it a little before using it.
On Gentoo systems, is the same as in CentOS or Fedora, adduser is just a symbolic link to useradd.
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