My suggestion is to buy a compatible ram kit of the size you need and keep the old ram as a spare.
am is sold in kits for a reason.
A motherboard must manage all the ram using the same specs of voltage, cas and speed.
Ram from the same vendor and part number can be made up of differing manufacturing components over time.
Some motherboards can be very sensitive to this.
This is more difficult when 4 sticks are involved.
That is why ram vendors will NOT support ram that is not bought in one kit.
Would you be at all upset if your new added ram did not work?