Im kinda new to this and i never even heard of this site which i regret not hearing about it and it seems you can have custom units to a scenario or custom map?
Yes you can add your own custom units with custom images and names and even give them different roles like example Musketeer Ground:Main atack , Swordsman etc you get it.
You can also make your own map , just upload an image and you can draw the borders yourself , create cities and countries wherever you like , adjust the income ,
reinforcments that the city gives and make it a port or not a port there are a lot of options so its generally possible to create any kind of map , even a dark souls map.
A good example of this is that there is a Game of Thrones map , you can check it out and see how it has been implemented into the mechanics of atwar