Yes, but it would come at the sacrifice of payload. Jay Utah will have plenty to say about it, but I think that nobody saw a mission for empty external tanks, so it just wasn't done. Obviously, they might have been converted to elements of a station, but at the expense of "paying" cargo. And outfitting empty tanks to hold crews...you'd need airlocks into the tankage, fittings for power, water, air circulation and so forth; anything you stick on to the tank before launch is just more dead weight. Anything you add later has to be done in freefall, probably in vacuum and it would be extremely challenging work. So we get ISS instead of the Orbital Hilton.