If you have a 16 zone controller, you can add (drag the sprinkler head) up to 16 zones.
Yes, the expander board can be powered by a 12V power source independently, but it's already wired up to the 24V source. Also, each of the terminal can handle up to 4A, and depends on how many lights you have it might exceed the amperage ratings. How many lights do you have and how do you want to program them?
Theoretically, it can be done, but we have not tested it and cannot support it.