RAM add

Apple won't do it. If you want to have the RAM installed, I suggest that you take the computer than authorized Apple Service provider to have the installation done. However, since it is so very important to install RAM that Macs will accept, you might want to make sure that the AASP gets the RAM (or you can purchase it) from only a provider that has RAM that works and supports it. Most of us here rely on and recommend RAM from crucial.com or mac sales.com. Their web sites will tell you precisely which RAM modules to install. If there are problems, they will replace them.

What year and model? If it's a 27", RAM installation is easy; admonitions about proper RAM always apply.

For the 21.5" model, it's far harder but a user with sufficient skills can do it. I'm not recommending you try.

Using a service tech will be rather expensive. So my question is why do you feel you need more RAM. If it's to speed up your iMac, that's not where to put your efforts. What problems are you having?

The only RAM Apple sells currently is for the expensive Mac Pro workstation. To follow up on well-stated ku4hz's advice, not all Apple Authorized Service Providers or independent tech will work on sealed-case iMacs like yours. Most customers balk a the two hours of labor (at andest US$80/hour) required to access the buried RAM slots in the 21.5-inch models. ku4hx is quite correct in advising to look for another cause of "slow Mac" than RAM.

History here regarding 21.5-inch iMacs is that about 95 precnt of the time, the "slows" are due to the under-spec entry-level mechanical hard drives Apple opted to install. You can spend a month's pay installing more RAM and see no improvement if the hard drive is the issue.