They could go on an external drive. What external ports have you got on the iMac?
Generally Apple's excuse for selling machines with low RAM is that everything is in the Cloud. I don't like In The Clod, hence I have a 2TB SSD in my machine. However, with modern port speeds, an external drive shouldn't be a problem. USB 3, USB C and Thunderbolt are all fine. 1TB of fast external USB SSD, c£140