Basically all you need to do is get the new FGD. That stands for something Game Data, and it tells Hammer all the entities that you can use for that game.
The Sven-Coop FGD comes with the mod, it's in Half-Lifesven.
To set up another mod, so you can map for more than one thing, choosing which to map for when you start up:
Hammer Options -> Game Configurations. Edit (highlighted), then "Add" in the dialog box
Choose mod from the drop-down list, then Add (highlighted) and add the FGD.
Note that Texture Configurations aren't kept for mods individually!