In this tutorial we are going to put a game in a different engine. For example: Half-Life 2: Deathmatch is a game running on engine 2009 but in Source SDK it's under the 2006 engine. This is easy to add it to the right engine.
Open Source SDK and select the engine you want. (in this example I use engine 2009)
Now double click on "Edit Game Configurations". And click "Add".
Go to your game directory (default in: C:\Program Files\Steam\steamapps[account name here][game folder] Select the folder of your game.
Half-Life 2: Deathmatch > hl2mp
Counter-Strike: Source > cstrike
When you selected the folder click on "Select" (#VGui_Sel.)
Now restart Source SDK and your game should be in the right engine now.
Open Hammer and go to "Tools" > "Options..."
Make sure the "Game Data files" are in the right location (eg. sourcesdk\bin\source2009\bin\hl2mp.fgd) You can remove the old one and click on "Add" to add the right data file.
Apply the changes and your ready to map in the new engine.
Don't forget to rate this tutorial, thanks.