A warm welcome to Mac-Forums, seedhre681.
Several points emerge:
You don't mention which Operating System your iMac runs.
8GB is sufficient RAM for all but the most demanding tasks - up to and including macOS Mojave
The addition of more RAM is a major challenge as harryb2448 pointed out. Unless you are comfortable stripping out the iMac, the risks outweigh the benefits.
Lastly, why do you consider the need for more RAM? A relevant question because if you are having some difficulty with your iMac, a modern iMac at that, then RAM is most unlikely to be the problem. Please tell us more and we shall be only too glad to help.
Ian