You wouldn't normally use the past tense. If you were born in the USA but you'd renounced your citizenship for some reason, you might say "I was from the USA." Usually stating your place of origin is done with the present tense. If you no longer live in your place of origin, you can qualify it somehow. For example: I'm from the USA, but I live in Canada now.

I can't think of a time when you would use this. If you want to use past tense for some reason, you could say, "I came from the USA." This could be used of both natives and people who visited (or lived in) the US and then left.

When is it possible?

What do you mean by "when"?