SG-17...you can, but in order to, you will need to "blank out" one of the city sets...in other words, make it a blank pink box...then you set the "City" unit to be immobile, and place it where you want the city...
Unless you mean making it a normal city...for that you'd have to ask RL to do it...or if he doesnt mind, I could try pulling it from a frame of the unit storyboard...but then ALL the cities for the culture group would look like Atlantis(as I have no desire to make an "Atlantis" type city set that grows with city size)
@Goldflash...if you are gonna "scold" the guy, at least help him out first...he made a legitimate request