adding more memory to my iMac (3GHz 6 Core i5)

"Configurable" means at the time of purchase.

No none of the 21.5" models are user upgradable. Some can painfully be upgraded, if you can find a third party AASP willing to do the work. The 2015 21.5" iMac's can not be upgrade and Apple will not do this type of upgrade.

To be honest it sound more like you might be low on free Disk space and not RAM.

To get a better idea of the issue at hand. Get and run EtreCheckPro from >https://etrecheck.com/en/index.html

Then post back here with the Report, as per > How to use the Add Text Feature When Post… - Apple Community

Is that the iMac Retina, 2017?

Yeah, that's not really user serviceable as it requires near-total disassembly of the computer.

Exactly what does that out of memory message say? It's likely not RAM, but storage that's the problem.

Are you experiencing any other issues with the iMac?