I believe you're looking to modify the skimmer to be an external skimmer, not a recirculating skimmer.
I would start by adding an o-ring to between the bottom plate and the skimmer body because it isn't a perfect seal. Then you just need to glue an input pipe to the pump input that sticks out the side of the skimmer and another pipe onto the output of the skimmer. Seal that hole around the pump input too (maybe a uniseal would work?).
As a side note, as someone suggested in another thread (very good idea) you could put an external skimmer into a small tub with a float switch so that if the skimmer overflows the float will cut power to the skimmer rather than pumping your sump dry (or causing the ATO to kick in and emptying your RO vessel).
Thanks to everyone for advertising for me.
