Coincidently, I had to do something similar the other day.
A friend of mine, who is in New Zealand gave me a $20 iTunes gift that he didn't need or want. I tried to redeem it as a UK Customer, but didn't work.
All you have to do, is sign up for a new account under the country you wish to use, as in my case, New Zealand. Login to that account, and redeem the code.
You then can download from it with no problems.