Plenty of options. You can run a stand alone refugium - how to plumb it depends on it's height, water level, horizontal location / orientation as compared to display tank &/or main sump.
You could run an external skimmer and/or external return pump to save space in your current sump.
You cold overflow into a refugium - which is then connected to the main sump which then returns water to the tank. Connection between refugium & sump can be bulkheads drilled through the sides of each and connected via PVC (diameter twice as large as return pipes and at least 2 of them) - or the refugium could be higher than the sump and gravity drain into the sump. What you want to avoid is trying to keep 2 pumps in sync volume wise - that's asking for a flood.
You could have a display refugium at the same height as your display tank - both drain into 1 sump and both are fed with just 1 single return pump - if you have the space this works very well IMO.
So - look at your space and think "how can I keep water flowing, using gravity and only 1 pump".
You could also do some sort of HOB refugium - although those tend to be way too small to do much good generally.
Some people just skip the skimmer and use nothing but a refugium area to export nutrients.
