Firstly Spanish TV is now digital. It is transmitted terrestrially. Sky boxes do not have a digital terrestrial tuner. Sky boxes only have an analogue terrestrial tuner. So you cannot receive Spanish TV using my Sky digibox in Spain if you connect it to a TV aerial in Spain.
Secondly, a Sky box cannot manage other encryption services. Most Spanish channels on satellite use a different encryption system to that used by Sky. So the Sky box cannot physically read a viewing card used to encrypt Spanish satellite TV.
A Sky box is designed to boot up and start to work using data from the UK TV satellites at 28 east. However, Spanish TV is transmitted from Astra 19.2 East and Hispasat 30 West, which do not carry the data required to boot up a Sky digibox.
If you want to watch Spanish TV via satellite then you will require a Canal+ dish, box and subscription.