Hello,
You can do it? Yes but is not recommended because the speed and timings of those tow memory sticks are different, the bios will downclock your higher Memory clock in order to match it with the other stick but timing will reaming different among a few other things.
I personally will not recommend you to mix them, use your 2400mhz stick and try to find a new stick at least with same seeped and timings (the ideal thing will be 2 identical RAM stick )