I think you can do it by running from master and explicitly specifying the actual db name in the query:
use master
select *
FROM your_db_name.dbo.Registos r
CROSS APPLY your_db_name.dbo.fnObtemCategoriaAviacao(r.PaisOrigem, r.PaisDestino)
SQL DBA,SQL Server MVP(07, 08, 09) A socialist is someone who will give you the shirt off *someone else's* back.