I have always added new LS to my tank with out any major problems , IMO it's a good thing to do cause it will give you more diversity in your DSB, you'll not need add much but just a cup now and then , as for the sand storm if one stops and thinks about this, think of the reef, can you say there is never a storm that kicks up a bit of sand now and then ? thats just the way of nature really, stirring up your existing sand bed is not good but IMO the sand from adding new would not be a big concern.