Is my warranty voided if I add more ram

Depends on the model and how you add it.

For the 5K iMac in your tag line, no, warranty is not voided. The 5K iMac has a hatch on the back to access the RAM slots and RAM upgrades are supported by Apple and do not void warranty.

See here: Install memory in an iMac - Apple Support

For the 4K 21.5" iMacs, there is no hatch, and upgrading of RAM requires it be taken apart. Unless its done by an Apple Authorized service provider, it will void the Warranty.

Phil0124 is 100% correct.

Adding that, Mac are seriously picky when it comes to RAM upgrades.

If you have a 27" iMac with a user upgradable RAM compartment, then I suggest only purchasing RAM modules directly from either OWCmacsales.com or Crucial.com.

If you have a 21.5" iMac that is not user upgradable, then I suggest having an AASP upgrade the RAM or replace it with a model that has more RAM.