With the information provided, the best that I can offer is a very big maybe.
There are many factors when it comes to mixing memory kits, and some details you left out, such as the motherboard and the timings of the memory, are very important.
Also, even if the motherboard should support it and the memory is nearly identical, there is no guarantee. Sometimes memory simply refuses to work together in a given system even if we can't find out why. Memory can be very finicky like that.
Also, the chances of it working often decrease when overclocking is involved, especially if the memory is what you're overclocking.
If you want to try it, then hey, maybe it'll work- but there is no guarantee that it will. With more information such as the timings, capacity, and voltage of the memory kits, the motherboard's model, and exactly what you plan on overclocking are given, then perhaps we can be more helpful.
Also, if you have a manual for the motherboard, it will probably have useful information for this.