Its do able, will it work is a crap shoot. Not knowing your system specs is a guessing game.
Say it works and your board will work in a hybrid single/dual channel mode. Your 2x4GB kit will bump up its voltage to 1.65v and drop its speed to 1600Mhz to match the single stick your adding.
This all hinges on if the new stick works with the other 2 and if your board like running 3 sticks