One way is to put the sand into plastic baggies and then lower them into the water where you can slowly spread them around. This is very tedious work but it doesn't create quite the mess. No matter what you do you will probably get some cloudiness. It really shouldn't hurt anything. I added some sand awhile back and I was able to get a bucket of sand and water above my tank. Then you can siphon the sand out with the water into your tank.
I'm sure you will probably get some cloudiness no matter what you do. I've dumped in sand straight from the top of my tank, making a huge mess, but it all seems to settle after awhile. I didn't notice any ill effects on the creatures. If you are worried about the fish you can always take them out for awhile.
