I know this has been answered, but I thought it might be important to add this:
Any FGD is only a tool for the Hammer editor.
An FGD lists all the game specific coded items that are available for use by Hammer.
Changing the FGD doesn't have any effect on the Game, it only changes how things are viewed in Hammer.
So if you want to add something to your FGD, then it must already be in the Game code for it to take effect.
Many of the original HL coded entities were indeed carried over to CS, but not all of them. Some of them were enhanced, some were not.
And as it has been mentioned above, You might be able to take a single entity and add it to an FGD, but it will only work in game, if it was coded into the gamein the first place.
Remember, it stands for Forge Game Data - Seventh