If you go to the Corsair support forum and ask the question, they will certainly say that they do not recommend it.
Adding ram afterward is not guaranteed to succeed, even if it is the same part #. Internal characteristics can vary considerably, and some motherboards are sensitive to the differences.
That is why ram is sold in kits.
It might work, and you would operate at the lowest common denominator of specs, and at the same voltage.
I suggest you get a single kit of the size you want, and market your old ram.