That's possible, but you would also need to keep the batteries charged when the grid is up, so the amount being siphoned off to do that would be noticed.
You would also need to do noticeable changes to the inverter, wiring and switch gear to be ready. Sure, it can be done, but who will risk getting caught by the solar company if they ever came out to check/repair the system?
A battery backup system to solar should be part of the complete system, so power you produce/save during the day can be used by you at night when the sun goes down. Unfortunately, very few people but that additional thought/construction into planning of their system...
They just wanted the "free" panels...