Anything and everything is possible if Bethesda wants something to happen, even if it's a direct contradiction of anything, or even everything previously established.
Sure, we expect a certain standard in keeping with an already established Fallout Universe, but, in the end, whoever owns the property can dictate any changes to that universe they want, and, make up any reason for the changes, or no reasons at all.
There's several quite plausible ways for introducing people, immigrants, invaders, and other such from other parts of the world. For one, unless the wind stopped blowing, anyone on the planet close to the ocean could sail a sailboat to anywhere else on the planet without any requirement for fuel, nuclear power, electricity or anything other than food and water.
As far as opening up world spaces outside the USA for the Fallout Universe, ther'e plenty excuse to do so, if anyone at Bethesda really wanted to, and there's plenty plausibile means by which anything of the sort could be introduced.
In fiction and imaginary worlds that don't really exist, anything is possible, like it or not.