Here's how you add sand to a running tank.....
Get a piece of pipe, 1 1/2 - 2 inch diameter, and cut it so that it is a little higher than the depth of your tank. Get a wide mouth funnel that will fit into the end of the pipe. Put the pipe in your tank with the funnel on top, all the way to the bottom, and now fill the pipe with sand. Oh, powerheads off. Now slightly lift the pipe while you move it about to distribute the sand. Repeat, repeat, repeat. Best to do with two people.
I would do this sand addition over a number of days/weeks, especially if you want a deep sand bed. Might also wish to throw an ammonia badge in there to make sure you don't have a mini cycle going on.