Yeah, you can modify the ASM afterwards....you're just adding a gate valve to the outflow with the intent of increasing the backpressure. This will increase the water level inside the skimmer, which will allow you to skim "wetter". Wet skimming removes more dissolved organic compounds, (DOC's for short) which are the impurities and pollutants in the tank...Some people prefer wet skimming, others dont.
It wont however reduce micro-bubbles unless you design it that way...and I dont know how to do that...
The fact that its designed in sump doesnt preclude it from being used out of the sump...you just need to modify it to work that way. You will need to modify the inlet, change the pump location, and modify the outflow.
Its a lot more work than just buying an out of sump skimmer in the first place IMO.
To be honest, the only reason I would make an in-sump skimmer, an out of sump skimmer, is because it didnt fit in the space I had.
But thats just me.
Nick