You mean the planets—let's not get carried away here. With that sort of hyperbole, Apollo-Soyuz was the first international mission to another star. As for the external fuel tanks, STS was already lofting a lot of excess baggage into orbit every time, namely the shuttle itself. The tanks were so stripped for efficiency that NASA stopped painting them to save weight. I doubt a metal shell with a few slosh baffles in it would make much of a space station. Skylab was essentially a modified fuel tank from the third stage of a Saturn V. If you've read anything about how difficult it is to work in a spacesuit (such as simply driving screws when repairing Hubble), you'd know that repurposing spent shuttle tanks in orbit is highly impractical.