 You can do it
I added several inches of sand to my 90 after it had been up for awhile. There are several other ways to do it than just dumping. I used a 2 inch piece of pvc taller than the water level to pour it into as I slowly distributed it around the tank. Easier if one person pours while another distributes. The slower you go the less mess you make. Another way is the old freshwater method of using a saucer or a large bowl. Any method results in some cloudiness, some more than other. I would be careful of covering any critters like snails, shrooms, etc. If you kill a lot of algae you can expect an appropriate bio-response (ammonia, etc.) Blow any dust off rocks after things settle, especially if you have coralline, which will die if covered.
Good luck  _________ Glass 90 and 110, skimmer in both 30 gal. sumps, 1150 GPH, plus a 75 gal fuge!! Yellow tang, several damsels, 2 lg flower anems, brown & rose BTA's (8" dia.) with 3 perculas, 5 ricordia, dozens shrooms, dozens of yellow, brown and green polyps, lots of snails, hermits, bugs and 3 inch worms. Mangroves, caulerpa, LR, LS, pics coming  | 
