Salvaging space junk (at least the bigger pieces anyway) to build onto stations has always been one of the plans, the trick is the fuel requirement to catch it. If ion or some other non-chemical engines are used it might even become practical one day (though ion engines would require robot vehicles or the ultimate couch potato for a pilot since they are so low thrust it would take a long time).
There is a lot of energy available in space and a lot of material on the ground, the expensive part is sending one to the other. Using what is already up there can be a big savings in launch cost.
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