Although commonly called a Converted Moon, the process can be done to practically any celestial body. It involves "equipping" a moon or other body with an engine system and other required systems for it to operate as a spacecraft. Most commonly a moon is equipped with a large engine system, powerful shield array for defense and life support, and some form of terraforming within the shielded area. For the most part they are methods for pre-FTL civilizations to go vast distances with all the comforts of home without resorting to crude generational ships.
Though primarily a way to travel interstellar space in style, another though less common method is to convert planets, moons, and asteroids into makeshift warships. This variation is most often emplyed when a Type 18 or more civilization is fighting a total war and they can't produce enough traditional ships to replace those being lost. So to keep pace, they shortcut the production process by equiping any suitable object with a shield, FTL drive, a potent power source, and various weapons. The result is a massive battleship with significant durability and noticeably cheaper production.