Q: can you add additional ram to existing ram on  iMAC 5K late 2014?

It good place to look up what is possible with your Mac is the everymac.com website.

Yes, you can.

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It sounds like your computer is a 27" model. If so then yes,  you can upgrade up to 64GB of RAM. Only purchase quality RAM, most experienced users on these forums only use RAM from either Crucial (buy it direct not a reseller) or from OWC (www.macsales.com). Both vendors have reputations of selling quality products at reasonable prices.

If your machine is a 21.5" model, then no it cannot be upgraded.

Yes, you can.

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