Think all you have to do is to log onto battlenet on the website (rather than inside sc2) and there should be two versions of sc2 registered to your cd key. Presume you've already registered your SEA account, just need to do it for the NA account. Then when you're logging into sc2, on the right hand side of the page you should be able to change the region, then use your newly registered NA account to log in.
unless they've changed something...that's how i did it.
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