This is exactly my problem with the whole idea of alien invasion. The technological resources needed to traverse star systems are so huge, and even then it's very unlikely any life-bearing planet will be all that habitable for your particular biology (the life there will have evolved for the conditions on that planet, which might very well be toxic for you). If you have such resources, it should be much easier to just build yourself some O'neal cylinders, or even terraform some more conveniently-located planets.