Yes, you can.... BUT beware that adding even the same model of RAM or different modules with similar specifications might result in system instability or even compatibility issues between modules.
If the RAM modules are bought together in a kit they're usually guaranteed to work as they're tested by the manufacturer. Combining them from different kits (even from the same model) is not guaranteed to work.
So, there's a risk of them not working together even if they're the same model... best thing would be to make sure you can return them. Also, test them thoroughly before settling on the buy.