XML wise there's an espionage mission info file (don't have the name off hand), that controls which missions are enabled in game and what their cost factor / base effect is. In the SDK these are pretty much exclusively the domain of CvPlayer.cpp for the functionality, CvGameTextMgr.cpp for the display functionality and CvPlayerAI.cpp for the AI capability to use missions.
Doing a search on destroybuilding or a similar pre-existing mission should help you find the relevant code.