How can I program a game using an already existing game engine?

Let's say I want to write a computer game using a program language such as C++, and that i want to use an already existing game engine; for instance the LS3D Engine used in Mafia or the GoldSrc Engine seen in Half-Life. Is this possible and how? I would like to know.How can I program a game using an already existing game engine?Yes, if the game engine is available. Then you download the SDK (software developer kit) and include it into your program.