I strongly suggest you don't do this, as sending a password through ftp which will gain root access is very insecure.
The only easy way to do this is to edit the user's entry in /etc/passwd to use the UID & GID of 0, but this will mean nothing on the server will differentiate between the user and the root user.
This separation of privileges is a very important part of the security systems inherent in Linux/Unix/BSD, and trying to subvert this will dangerously weaking your server security.
Jeff