From what I'm reading, one RAM module is soldered, and there is one additional bank, meaning the only option is, in fact, to add a stick.
@Merlijn_1: Try to match the memory that's in your system as closely as you can. Even if you can't match the brand, match the speed. You can find out specifics about your current RAM by downloading and running HWiNFO64, then go to the memory section for details.