You can either use your existing iTunes account and convert it to a US account or create a new iTunes account. The key here is to redeem a US iTunes gift card. This will enable you to pay for the HBO NOW subscription.
- Purchase a US iTunes gift card. You can use this site or search on ebay
- Open iTunes on your computer
- Once logged in click your name top right and select redeem
- Enter the gift card code
- Your account will change to a US account and you will need to enter US billing data. You can use http://www.fakenamegenerator.com/ to get a US address.