Welcome to the forums my friend!
Mixed RAM modules are a 50/50 gamble. So they are not guaranteed compatibility,. Not saying they never work, but it's a gamble, so there is every possibility you can upgrade the RAM and they won't play nicely together.
Tht aside, you can do it, however you'll probably find that either your other RAM will downlock to match the new RAM, or it might not work at all. Frequency / speed etc is just a way to minimise risk, not eliminate it.
So best practice is to buy completely new RAM.
Next to that it's getting the same model RAM, or at the very least RAM that is an exact match in specs.
There's no way of telling if it will work until you try it my friend unfortunately.