Quote: "Simply put, I want to know how to make a bigger more detailed world than what FPSC will allow."
Welcome to the club
But seriously, there is not much you can do about it. You can indeed import maps into FPSC which are generated with other programs but NPCs will not be able to move on the surface then.
In addition the premade landscape will not help that much because every dynamic object (NPCs, Monsters, scripted entities) eat up memory in large chunks too.
If you want to create something like Fallout, then you probably should rahter start modding Fallout itself.
Or you do what everybody else does and stay in underground, sparsly lit, cramped rooms. Which isn't so bad - it worked for Metro 2033 at least
In case you find my grammar and spelling weird ---> native German speaker ^^