How to buy songs from another iTunes country store via credit card?

Hi there,

I registered for the iTunes Store Germany, paying method credit card. Now I'd like to buy some spanish songs which are only available in iTunes Store Espana.
By starting the buying process I receive a message, that my account has only access rights for the german store. -.-
What can I do to buy these songs? I even don't understand the sense of this restriction.

Hope to find a solution here.

Thanks,
Tobias