declare
    object_exists exception;
    pragma exception_init (object_exists , -00955);
begin
	execute immediate 'ALTER TABLE EVENTO_EXTRA ADD (COO VARCHAR2(6))';
	exception when object_exists then null;
end;
/

declare
    object_exists exception;
    pragma exception_init (object_exists , -00955);
begin
	execute immediate 'ALTER TABLE EVENTO_EXTRA ADD (GNF VARCHAR2(6))';
	exception when object_exists then null;
end;