You would need to add a column

ALTER TABLE userlog
  ADD( user_id number );

create a sequence

CREATE SEQUENCE user_id_seq
  START WITH 1
  INCREMENT BY 1
  CACHE 20;

Update the data in the table

UPDATE userlog
   SET user_id = user_id_seq.nextval

Assuming that you want user_id to be the primary key, you would then add the primary key constraint

ALTER TABLE userlog
  ADD CONSTRAINT pk_user_id PRIMARY KEY( user_id );

If you want to use the sequence to automatically add the user_id when you do an INSERT (the other option would be to specifically reference user_id_seq.nextval in your INSERT statements, you would also need a trigger

CREATE OR REPLACE TRIGGER trg_userlog_user_id
  BEFORE INSERT ON userlog
  FOR EACH ROW
BEGIN
  :new.user_id := user_id_seq.nextval;
END;

SAMPLE PPPPPPPPPPPPPPPP

ALTER TABLE TEST_EMAIL

  ADD( NEWID number );

CREATE SEQUENCE NEWID_seq

  START WITH 1

  INCREMENT BY 1

  CACHE 20;

UPDATE TEST_EMAIL

   SET NEWID = NEWID_seq.nextval