BEGIN
  
  EXECUTE IMMEDIATE q'[alter table schema.CLIENT add ( 
                                  CODE_RISK_GROUP  VARCHAR2(4000 CHAR) DEFAULT 'XNA',
                                  DTIME_RISK_SCORECARD_TYPE  DATE DEFAULT to_date('01.01.1000','dd.mm.yyyy')
                                   )]';

  EXCEPTION
    WHEN OTHERS THEN
    IF (SQLCODE = -1430)
    THEN
      DBMS_OUTPUT.PUT_LINE('Already done, do nothing');
    ELSE
      RAISE;
    END IF;
    
END;

DECLARE

    v_xna                        CONSTANT VARCHAR2(10 CHAR) := 'XNA';
    d_def_value_date_hist        CONSTANT DATE  := '01.01.1000';

BEGIN
  
  EXECUTE IMMEDIATE 'alter table schema.CLIENT add ( 
                                  CODE_RISK_GROUP  VARCHAR2(4000 CHAR),
                                  DTIME_RISK_SCORECARD_TYPE  DATE,
                                   )';
                                               
  
  execute immediate 'UPDATE schema.CLIENT 
    SET CODE_RISK_GROUP = :1,
        DTIME_RISK_SCORECARD_TYPE = :2' using v_xna, d_def_value_date_hist;
    COMMIT;
						  								  
  EXCEPTION
    WHEN OTHERS THEN
    IF (SQLCODE = -1430)
    THEN
      DBMS_OUTPUT.PUT_LINE('Already done, do nothing');
    ELSE
      RAISE;
    END IF;
    
END;