If you couldn't add additional maps, the whole point of having an SD slot would be moot. No one really wants to look at their photos.

Garmin even sells other 2xx series devices that are more or less identical, but with different or additional maps depending on the model.

However, it doesn't have enough memory to calculate a route from PHX to LHR - I tried. I wanted to see if it would give a Google Maps-like "Swim 2000 miles" step. Slightly unrelated, it is supposed to use ferry routes available in much of the world as viable route options.