Electrical – Unusual three-way+ light configuration

If both switches are of the type 2 configuration in your diagrams, then no. You need unswitched power from somewhere to either the new light or new switch. For example, if either of the switches in the 3 gang box were of the type 1 configuration, then you have unswitched power and can add another switch.

To do what you want you would need to run another conductor from where there is unswitched power available.

I would sell this kit and go with another one that is compatible with LED lighting and your wiring scheme. This one is not. If you're not into that ...

Looking at the manual for this 45607 dimming 3-way... each smart device needs always-hot (which I show as black here), neutral and ground, which is fairly typical of smart devices. They need power (hot+nuetral) simply to function.

In this family, only the 45607 main actually switches the light, on a "switched hot" line (red here). The remote merely sends a data signal to the effect that "the human requested on/off/dim", and even though these are supposed to be wireless devices, they have a data-signal wire between them to communicate (yellow here). There can be an unlimited number of 45610 remotes -- anywhere they can get hot, ground and that yellow signal wire. I assume they are misusing ground as neutral; Code actually requires you bring a neutral anywhere you bring power, so I drew it in.

And you can see where that fits pretty well with most 3-way wiring. The black and white can be used for always-hot and neutral, and the red wire can be re-tasked as the yellow signal wire. Usually.

It does raise some interesting questions about the 45610 remote. Why couldn't it run on a couple of AA batteries? Why can't it use wireless/radio to communicate with the 45607 main? And in fact, the ZWave system has many manufacturers, another one may make a remote which does exactly that. That is worth looking into.

Now, in your setup, power feeds into the lamp itself. This leaves you with a wire problem. Here it is.

See, you can hook up the 45607 no problem. And it will control the light just fine. But now you've used up all your wires between lamp and main. So when it comes time to add the data-signal wire over to the 45610(s), you're out of wires.

Here's the thing. The yellow wire is a data signal wire. It's not a power wire. Check with the manufacturer but you may be able to use intercom, doorbell or thermostat wiring for it, which has relaxed rules for installation. You only need to get from the main to the lamp. You can re-task the old red wire once you're there.

There's one last way to make this work, but it's crazy. Put the 45607 main controller inside the fixture. Use two 45610 remotes, one at each lamp position. You'll have enough wires for that.