Prepositions don't always translate the same way when translating from one language to another.
From what I understand, היית would be a stative verb, and would require the preposition ב. On the other hand, if you wrote הלכת, נסעת, טסת, טיילת and so on - verbs of movement, then I guess you would use ל. But maybe someone can correct me if I'm wrong.
From what I understand, הלך is used for going, in the sense of walking, so I don't think this would work.
Actually, now that I saw closely what you wrote, you actually wrote - "have you walked IN America already". This means something completely different than the sentence we have here. I guess you meant to say "הלכת לאמריקה כבר" - that might be translated - going (walking) TO America.
Israelis I know, being accustomed to calling the United States "אמריקה" when speaking Hebrew, use the word "America" when speaking English in cases where an American would say "the United States" or "the US". Thus they greatly overuse the word "America" in English and don't sound like native English speakers. (At least not like Americans.) For related reasons, I think "Have you been to the United States already" and "Have you already been to the United States" should be accepted here.