I wanted to reply to this relatively quiet thread to share my own experiences and how a buddy of mine who is an electrician helped me out to get this set up. In my situation, I had a neutral at one switch, but not at the second switch (considered the add on switch going forward). The products that I used were the GE Z Wave Toggle Switch and the the GE Add On Switch. I ran into the same problem that spawned this thread, and luckily my friend was able to help me out.
If you look at the original installation for the three way switch, you have the power coming from A and out to the light through C. (see my own reply to see diagram)
Take note that in the Add On switch instructions, this is nearly the exact same thing, except that the connection to the light happens at the second switch box (at the add on switch).
(see my own reply to see diagram)
What he did was to connect the main switch directly to the load (to the light) like the original instructions, then pigtail T1 to the neutral. At the add on switch side, he capped off the load to the light (instead of connecting it to T1), and then connected the jumper for neutral to T1. The rest of the connections remained the same with T2 going as the Traveler.
(sorry about shoddy diagram, I used Paint to stitch together the two manuals and then edit to show what he did)