Adding LIVE sand may be detrimental to an established tank, in your case, I'd add dry sand, an established tank wont take long at all to liven up the sand.
Now, when adding, rinse the new sand thoroughly in RO/DI water, to get any fine particulate out, use a cup, scoop up the wet sand, and gently lower it to the bottom of the tank, that should hold it together as the sand is heaver than the water, then gently dump it onto your sandbed.
Now, more importantly, how deep is this gonna make your sand bed? Why are you wanting more sand? And how often to you disturb/mix up/clean your current sandbed. If your current sand bed is well cleaned, and often disturbed, you should be ok to mix the new sand in, but if not, before adding more, I'd wanna slowly beginning to clean out your current sandbed, maybe 10% of the bed at a time, so you dont release a bunch of crap from it into your water.
Hope this helps!