Outside the standard workbenches that are bundled with every FreeCAD installation, it is very easy to add other workbenches that extend FreeCAD functionality. At the moment, since FreeCAD is very young, there is almost no other workbench availible. But FreeCAD has been made extensible right from the start, so adding new workbenches is very easy, and in the future several new workbench should appear.
Extra workbenches are delivered in a folder, that must simply be copied into one of FreeCAD's Mod directory. The Mod directories are scanned everytime FreeCAD is started and the availible workbenches automatically added. There are two ways of installing:
Extra workbenches, or modules, can be installed inside your FreeCAD installation directory, so they will be availible to all users. Depending on your system, you might need administrator or superuser privileges to access the FreeCAD installation directory.
Copy or move the folder containing your workbench to the "Mod" directory that lies inside your FreeCAD installation directory. On Windows, it will probably be something like C:\Program Files\FreeCAD\Mod, on Debian-based systems something like /usr/lib/FreeCAD/Mod
Workbenches installed that way will be availible only to you, and not to the other users of your system. But it will not require any administrator or superuser privileges.
Copy or move the folder containing your workbench to the "Mod" directory that lies inside your FreeCAD user directory. On Windows, it will be something like C:\Users\your_user_name\Application Data\FreeCAD\Mod, on Linux systems it will be ~/.FreeCAD/Mod where ~ is your user's home directory. On Mac OSX the "Mod" directory should be ~/Library/Preferences/FreeCAD/Mod. One way to get to the FreeCAD preferences directory on Mac is to use the Finder menu item Go->Go to Folder... and then enter "~/Library/Preferences/FreeCAD".