Adding more LS
would be fine but do not put it in all at once. You would do better to add only ˝" or so at a day to allow the "critters" in the sandbed to migrate to the surface.
I find using a small plastic cup best. Allow it to sit just a hair below the surface of the water in the tank until it fills slowly with water. The gentley ease it down to the bottom and ease it out. Don't dump, just let it slide out easy. It would be a good idea to turn off the powerheads in the area you work on. The few minutes it will take won't do any harm but will lessen the murkiness.
No need to worry about the fish. I swapped out my CC
for sand with a "full" tank (rock, corals and fish) and did not move a thing (or lose any either