Can I mix 3rd party RAM with Apple RAM?

I have a late 2014 Retina 5K 27-inch iMac that came with 16 GB of Apple RAM.

I now would like to upgrade it to 32 GB. Can I just add 16 GB of compatible RAM (OWC, Ramjet, Crucial, etc) or should I replace the 16 GB of Apple RAM with 32 GB of compatible RAM? I have Google'ed the question and seems that some have had issues with trying to mix 3rd Party RAM with the original Apple RAM. Also, I would like recommendations for 3rd party RAM providers. There seems to be a wide range of prices for RAM that claims to be compatible with my iMac.