I periodically add sand to my tank like this:
I bought a large pvc pipe from hardware store (1 1/2 inch diameter I think.) and a plastic funnel. I just stick the pipe down into the bottom of my tank, put the funnel on the pipe, and pour away (works better with dry sand, but either will work.)
According to Fenner, a DSB of 4 inches or more provides the proper habitat for anearobic bacteria (oxygen poor bacteria) in the lowest levels of the sand bed that turns nitrates into nitrogen gas and oxygen.
There are a couple of dangers to a dsb. Be sure to research it before you do it. Persoanlly, I love my 6 inch sand bed.