well... after doing my post I've found this..

http://www.firebirdfaq.org/faq322/

and I did this

CREATE PROCEDURE PANIO_TERMINADO (
in_id fk_id,
in_etapa codigo)
returns (
out_fecha fecha_hora)
as
declare variable loc_sql descripcion;
begin
loc_sql = 'select ' || in_etapa || ' from pedidos_control where id = ' || cast( :in_id as varchar(10));
execute statement :loc_sql into :out_fecha;
suspend;
end

It's OK? or it can be improved? Thanks again!!!!

-sergio