With a lot of effort! (And even more patience.)
They will get sucked in the return pump and migrate to the main tank, so I wouldn't waste time harvesting them on a regular basis. Meanwhile they are just back their eating detritus and being a good cleanup crew.
However, if you get a new fish that is not eating prepared foods yet, harvesting amphipods from the main tank to feed them in QT can be a lifesaver for the fish.