create mail spool for user

[mahesh]

I've seen many topics about use useradd command without creating a mail spool in /var/spool/mail/$USER.
How can I create a mail for a User that already exists on the system?
Example:
root doesn't have a file in /var/spool/mail, so when an email is sent to root, it goes to /var/spool/mail/nobody;

I would like to create an email file for root. Does anyone knows how can I do it?