Is there a way to make structures randomly appear over a period of time?

I am thinking of adding Immigrant camps to my structure mod and was curious about the ideal way to make camps and perhaps ships that appear randomly outside of my structures which will allow Immigrants into my cities. I would like to have either small camps, wagons, or ships appear every which would add new citizens to structures I build and want the structure to appear randomly and disappear randomly. Just curious about how one would make such a thing happen.