If exactly the same kit that you already have it should be okay, only way to know for certain is to purchase the kit and test with what you have. As others have said, even unmatched sets from different manufacturers often work fine.
The theory is that a truly matched set (i.e. sell what you currently have and replace with a larger kit) has been tested with all dimms to make sure they work well together. I think things like the chips may be slightly different even in the same product as manufacturing might change slightly over time which is why they say in a perfect world you use a fully matched kit.
I've often bought a pair of kits for video edit suite builds where you need shed loads of ram as 64Gb or larger kits don't really exist. They've always worked fine although they are obviously likely from the same batch as they were purchased together.
I'm waiting for a second hand 32GB kit of corsair dominator quad channel 1600 to arrive from that auction site to go in my x79 build. I'd be VERY surprised if it didn't work fine with what is already installed. Can't see x99 being any different as they have been saying the same thing ever since dual channel RAM was released all those years ago...
E-I
