declare
    object_exists exception;
    pragma exception_init (object_exists , -01430);

  begin
    execute immediate 'ALTER TABLE CAJA_DET_PAGO ADD PAGAMENTO_ID_TROCO_SIMPLES VARCHAR(100)'; 
    exception when object_exists then null;
  end; 
  /
  declare
    object_exists exception;
    pragma exception_init (object_exists , -01430);

  begin
    execute immediate 'ALTER TABLE CAJA_DET_PAGO ADD CPF_TROCO_SIMPLES VARCHAR(15)'; 
    exception when object_exists then null;
  end;