Change Create Table Grammar

Hello everybody
I'm a new user in PostgreSQL, I want to add new column in a table where I
create a new table automatically ...

In fact, I want to do something that when the user enters the command to create
the table without the user realizing it, add a column to the table and there
hash of each row is entered to be maintained ...
While, I do not know where to put this command and what should I enter a
command. Please note that the information you give me on this ...

Thank you ...