Hola!
Soy Belga y vivo en Bruselas, pero estuve viviendo once anos en Espana y conozco bastante bien la problematica de la importacion...
As far as I know there is only two european countries that will import Big Dogs easily: Poland and Lithuania.
In other countries you will need to do an individual type approval which is a long and costly process, you need to have the tach, headlamp, rear indicators and exhaust changed, plus you need the plate mounting to be placed on the rear fender, and then there is the gamble of managing to have your engine running lean enough for the emissions test, all pretty difficult...
I suggest you get in touch with someone that will do the importation for you in Poland or Lithuania and then, once you have the european papers it's pretty easy to have it registered in Spain.
Hope it helps,
PM me if you have more questions...