Hello Hikary Katsu
Yes you can buy a identical single stick and install it. Most people in the industry will say that you should buy memory in a kit (16GB kit 4x 4GB or 2x 8GB) but in over 10 years I have been in the PC industry I have not has any trouble with mixing kits. You might run into problems if you start to mix brands. I have ran into certain brands that don't get along together but others do.
The reason most people say don't mix kits is because the ram in kits, are made and tested together so there is less chance of failure, but if you run memx86 (http://www.memtest.org/) and it passes after 5 passes then you shouldn't have any trouble. I think also most retails say you have to buy a kit is because you will need to buy more. Like in your case you have 2 sticks of ram already installed and according to retails they expect you to replace all of your memory and not offer a trade-in. Any business that is offering that, I would be questioning their motives.