As long as the ice blocks are covered they should not heat up and therefore cracks are unlikely. The igloo idea is really an outer covering and there would be a separate sealed area inside. You might even seal the whole ISS inside an igloo like this. That would be the one that needs to be crack free as that would contain the air and occupants. The igloo would also act as a meteor shield, so cracks might occur if there was an impact. An ice block could be recast if it was damaged or just melted to use the air, and another block put it in its place.
Most ideas about capturing asteroids have a lot of merit. The idea of this is to tide us over until we get to that stage. Then the operation you describe would be a good idea.
The beauty of this system is that even though the materials are not ideal for space they are pretty useful. Any alternatives have to be flown up from the surface which is very expensive.