Hi there! I'm from Michigan, I hope I can offer you some help. If the dumped duck is in no real danger, like injured or frozen to the lake. Sometimes these rescue places are just too overloaded with injuries and ducklings that can't protect themselves to take on any thing that doesn't need immediate attention.
Mitigating birds are illegal to own/catch unless your a licensed wild life rehabilitation center. They are federally protected. Domestic dumped birds, to stay complacent with the local town and to stay in good graces, I would seek approval from the town/city they are located. Someone has to own this pond, it could privately owned or the city owns it. Trespassing would not be a fun thing to get nailed with.
To catch this guy, I would win him over by providing treats from your hands. Doing this daily, you how ducks like routine. It would be a slow process but once he earns your trust, you might find easier to get him. You have time before winter hits.
Yes bringing this guy in would not be a good ratio for your current flock. But you might be able to rehome him, but you might not. That's a bet you will have to really think about. Do you have a way to keep him separated until you can rehome him?
Quarantine rules (for me) go as - 40 days and 40 feet away from your current flock. If you have a vet that could do a poop sample on this guy, you could get a better understanding of what your dealing with. He might be healthy, he might not. This might cost you some money, but I would rather do this then take the chance of getting my entire flock sick.
I have found a few rescues here in lower Michigan/Ohio border. I'm on the Lake Erie side. I'll make a list in the morning for you. This iPad is not really friendly with this app, my desktop at work is better for grabbing resources.