Just off the top of my head:
1. Sand could contain dead organisms thus creating a small ammonia spike if not cycled properly.
2. If new sand is added on top of current sand it reduces the area active with live beneficial bacteria since you just buried a whole lot of surface area. It also covers holes dug by micro organizms living in your sand.
3. If you decide to fix this by 'mixing' the sand you run the risk of overstirring the sand and releasing trapped nitrates/ammonia that hasn't been fully broken down into the water column causing a spike.
.... so basically you could probably manage as long as you assure that the sand is fully cycled (by doing it in a sep. container) and won't cause any kind of spike. If your adding non-live sand then just be aware that you may cause problems by burying the holes dug by your micro organizms currently living in your sand and reducing your exposed bacteria laiden surface area for a period of time while it seeds your sand.
