Create an addon

Is it possible to create an addon for my own use? I want to, say, build out a map and NPCs and such for a city, and then, if my party comes across that city in some campaign, drop it all in at once. 

Only by publishing one to the marketplace as such... and that comes with all the copyright implications for any text/art etc. What you could easily do as a Pro user is set up a separate game with all the content you might want to import and use the transmogrifier to bring it across as needed. Won't be as quick and easy as an add-on but if your holding game ONLY has the content you want it wouldn't take too long to drag/drop it all across to the main game.

Bummer. Thanks for the answer.