It can be done from an iOS device (but not from the Mac/Windoes iTunes app)
In the iOS App Store app, make sure you're not signed in, then try and buy a free app from the US store.
This will prompt you to either log in or create a free account.
Choose create account and fill in your details.
When it asks for payment details, the option "None" should be there. Choose this.
Add any other details needed to finish creating you account and you're done.
You should then be able to top up your account using gift cards.
This was working last week as I did it to download a free film that was, for some reason, US only