(split this into it's own thread so you'll have a better chance of responses)
there are a few ways. I would say get a long piece of PVC
tubing, and pour the new sand down through that with the end of the pvc
down on top of your existing substrate. this will minimize the amount of sand that gets blown around while you're doing it. No matter what you do, you will get some sediment that gets blown around, all you can do is to try and minimize it.