How about adding them as "course points" in the Garmin Connect course? Go to your course in Garmin Connect web and edit it. Select "Add Course Point". Type "Generic" and give it a meaningful name.
Is that what you mean or do you mean adding them as POI's to the map/database?
There is such a tag as a Charging Station now but i'm not sure how wide spread this would be in the map data yet.
