Hi!
I am not sure I fully understand your issue - it sounds like you have an existing LightMix set up, and then you add a new light and want to add that to the LightMix setup. If so, then you wouldn't use the automated process to generate LightMix layers, as you already have them there (the automated process is for making it easy to handle the initial set up, when you will likely have many lights and don't want to add them all manually).
For just adding a new light, simply manually add a new LightSelect layer and assign the light to it, which leaves your previous LightMix set up intact.
If that isn't what you were asking about, though, let us know :)