Yes SolarSeaman, I know the ET isn't designed for that, because it wasn't meant for such things. But as I mentioned in the OP you could design one
tank a bit special and I doubt it would add so much weight that it wouldn't be able to reach orbit.
Ngchunter: sorry but the links don't work on my computer even if I copy them in google. But nevermind I know there is a page on Astronautica about
that. You mentioned the possible problem of insulation. You could put the giant surfaces for heat radiation outside the ET itself, like they have on
ISS. About the work: You could build the interior after the Shuttle landed. Just establish life support system and a few beds

and the astronauts
would work. Yeah I agree there is a lot of work, but I doubt the cots would be higher than for the ISS and as said before, volume is huge....

I
totally agree, the idea should be polished.
Greets