If you aren’t Jewish, you get to choose Ashkenazic or Sefardic — there are a few Western European Sefardic communities including one in New York and one in Philadelphia. But you need to be part of a Sefardic community if you want to convert Sefardic. And be warned that it is MUCH easier to be Ashkenazic here in America as all the major kashrut organizations accept Ashkenazic leniencies that aren’t accepted in normative Sefardic halachic rulings.
If you are already an Ashkenazic Jew, it is a machloket as to whether one can ever become Sefardic. I personally have a psak halachah from a Sefardic rabbi that I can — if and when I move to a Sefardic community. But I have also heard at least one prominent Rosh Yeshiva say that it is not possible, ever.