Most recirculating skimmers work this way, where they can just be plumbed into any available source of water, or even to a tank draink.
However, skimmer design precludes their installation in a pressurized line. What I mean by that is that water coming out of a protein skimmer has to drain away by gravity. You cannot force water through a skimmer by use of pressure, such as by plumbing it into a return line or anywhere where the water leaving the skimmer is pumped to a level higher than the skimmer's collection cup. The skimmer's effluent must drain horizontally or downward into the tank or into a sump. There is no way to pump water "up from" a skimmer.