Tee in to your plumbing like this
Pumps in to your tank
Than just get a cheap external overflow to go back in to your sump
Or you can drill a tank like this and just have it drain back in to your sump. But put a screen so nothing can plug it up. I no longer have this tank online. It's just keeping some liverock alive for me.
As you can see it's a cheap easy way to plumb as I just used some odds and ends I had around