Source games and mods may be used in synergy using Steam Mounts (SteamContent.txt) which allows you to use models, materials, and sounds from other source games. Settings can be changed from within the game:
IMPORTANT: Do not mount a game or mod you do not have installed! Should you do so, or if you are having problems, start the game with the parameter: "-resetmounts"
SteamContent.txt can also be modified with any plain text editor. The appId parameter should only be defined for steam games, not mods; you can find a list of appId's here. Only mount steam games that run on the source engine!
Adding a mod requires using the following format for the appPath: