Q: can consumer add memory easily to the iMac/

yes there is a door at the bottom of the screen that gives access to the memory.

See iMac: How to remove or install memory

You can add or replace ram to imacs. OWC sells various configurations and here's their instructional video on how to do it on a 2011 iMac as an example of how east it is to add.  I assume they don't have one for 2012's because nothing changed from 2011.

I always recommend OWC for memory but there are other suppliers.  They are very reliable and test their ram.  It may add a few bucks to the price but you can be sure it's proper memory for your mac.  If it fails they will replace it (but I think so will all other ram suppliers).

Here's the OWC link for iMac ram.  You didn't specify your model year so this link is to all the iMac ram selections.  Click on the year for your model.

According to the specs, the memory is consumer accessible on the new 27 in. iMac, but adding memory to the new 21 in. has to be done by Apple.