Most people recommend quarantining new ducks to make sure they are healthy and don't have any diseases that your current ducks can catch. After that, lots of people have good luck with having a fence/barrier up between the new ducks and the your current ducks so everyone can see each other without being able to touch. That's what we did with our new drake, with supervised interactions until we decided it was okay to have him with our girls full time.
I'd also recommend getting at least two new ducks when you do so they aren't lonely while waiting to be with your current ducks. Henry was supposed to have a new female with him but she died unfortunately so he was by himself and absolutely distraught about being alone.
All of this is assuming that you are getting adult ducks. If you get ducklings, I think the usual advice is to wait to fully integrate them with your current ducks until the ducklings are closer to being full grown.