Adding an Electrical Outlet when original is in conduit (what do I do with the ground?)
It's apparent the contractor is using the conduit system as a ground. You didn't mention what you are using for the extension but I am assuming it's some sort of approved cable. You can attach the ground to the box the existing receptacle is in. You will find a threaded 10/32 hole in the back of the box. You can purchase a ground screw at just about any hardware store. 

I must caution you that if you are in a municipality that only allows conduit systems in houses you should install expansion's in like systems.

Hope this helps, good luck.