Populate a column in table with system user by default.

Hello Experts,

               I have created a table T1 with four fields. I want to add additional column to this table say C1 that contains the system username by default.

For this I wrote an ALTER TABLE statement as - alter table "SCHEMA"."ZDB_TEST2" add (created_by varchar(20) default system_user);

But I am getting syntax error in this case. Please guide me in order to resolve this.

Thanks in advance,

Dnyanesh