What mobo/CPU, generally if they boot together it will be to 1333 (guessing the mobo's default), then to try and run at 1600 or better will require manual setup
It won't blow anything, but the system will likely not boot up, or will work fine, maybe with issues like BSoD, adding non paired sticks is a gamble, and unless its exactly the same model, there's every possiblity it'll not work out.
Either you will get pleasant beeps and PC won't boot or you will end with PC booting but with both RAMs set at stock clock i.e 1333MHz. It is not advisable at all. Better grab similar stick like your existing one or you may face compatibility issues.
What mobo/CPU, generally if they boot together it will be to 1333 (guessing the mobo's default), then to try and run at 1600 or better will require manual setup