Not at all, really. Simply build them, then export the prefab noting it's exact coordinates, then add that prefab to the prefabs.xml file with those exact coordinates.
This is a custom 8k world I created using Unity/Gaia as well as NitroGen to place roads/poi's. I've manually added two POI's (Modern Art Gallery by Topminder and Pille and Quiet Pines by Hernanxx). DOWNLOAD LINK
Preview Pic of World:
Nitrogen Preview:
3D Preview (more or less, it's slightly different in game than it is in this Unity picture):
To install, you simply unzip the file into your game folder. The result should create:
/data/worlds/guppysisland
/data/prefabs/ ...the two prefabs
To use, simply start the game, choose Guppys Island and give it a unique game name, and voila.
