From version 1.37 the app has ability to use custom maps.
To use your custom maps you need to setup somewhere in the internet folder including index.json and directories by the names defined in index.json. Each directory then should contain files map_.jpg and map_info_.json where * is number specified in index.json.
Then just run the app and input the address to the folder (including index.json and directories) to the settings.
You can get more info here http://junkcode.net/forum/viewtopic.php?f=14&t=19