I currently have a 3 way light switch in my stairwell. One at the bottom and one at the top of the stairs. I recently added a light fixture to the bottom of the stairs but need some help wiring it up. I have been rewiring the whole house but the 3 ways a bit confusing to me. Power comes in from the light fixture at the top of the stairs. Here is a diagram of what I have and what I am trying to do. Any help would be great. 
Can I tie into the wiring leaving the original light before it reaches the switch on the left?
Best Answer
Smart switch; change one cable
The cable from the existing lamp to the nearer switch (upstairs?)... you need to change that to a /3 cable (red white black).
You can extend the cable to your new lamp from any switch box. When you do, use colored tape to re-tag the black wire as a red wire.
Now we're going to fit smart switches that allow wireless connection of remotes. These smart switches cannot require a wire for comms between them, as we lack any spare wires.
Now we're going to change the color codes. As it happens, the existing color codes will suffice in all locations:
Your master smartswitch attaches to black, white and red.
Your remote smartswitch attaches to black and white only.
Your original lamp connects to red and white.
Your new lamp attaches to red and white in the box nearest it.