The service is available, but....there is always a but, the service in theory is directed to companies and at a national level.
If I've understood correctly, you are looking for a P.O box and mail redirection. The national post service has those for companies at a national level...
P.O box---> Apartado postal
Mail redirection ---> Reenvio postal
Both in
www.correos.es , click on Inicio, empresariales on the list and finally click on Apartado Postal. Only in spanish I am afraid.