Im pretty new to Unity, and I’m making an FPS.
A few of the maps require me to make a flat path down a hill. like shown in the link
http://trampingtracks.co.nz/pics/summit%20(22).jpg
Is this Possible with Unity’s terrain editor or do I need to make this Externally?
You will have to do this externally. Another option would be to download an editor extension like EasyRoads 3D (http://unityterraintools.com/tools.php). This is a really nice tool for making roads on a Unity terrain object. However I don’t really know if you could make stairs with it (like in your picture)… Take a look at the movies they uploaded to see if you can do anything with it.
Hope this helps