declare 
  column_exists exception;
  pragma exception_init (column_exists , -01430);
begin 
  execute immediate 'ALTER TABLE BOLETO ADD FECSEGUNDAVIA DATE';
  exception when column_exists then null;
end;
/
declare
begin
  execute immediate 'UPDATE BOLETO SET FECSEGUNDAVIA = FECMODIF WHERE INDSEGUNDAVIAIMPRESSA = 1';
  exception when others  then null;
end;