My understanding:
The grid provides a stable reference voltage and frequency, and the Powerwall inverters can operate in grid-tie mode by matching this reference. Likewise the grid-tie inverters on a PV system.
When the grid is down, something needs to provide a stable reference voltage and frequency, and take care of ensuring that all times, power produced exactly equals power consumed, i.e. to balance this mini-grid. The Powerwall inverters can operate in standalone mode to do this, allowing any PV grid-tie inverters to sync to it and generate power. If the solar output is greater than can be handled, the Powerwalls shift their generation frequency outside of the PV grid-tie inverters' sync window to get the PV to shut down.
Now if you want to add a regular generator to this mini-grid, I don't think you can AC-couple the generator. I don't understand enough of the innards of a generator to know how a generator normally balances a mini-grid consisting of it plus loads. But I'm pretty sure however generator balancing works, it won't play well with Powerwalls. How is the load supposed to be divided between the generator and the Powerwalls, and how would that happen?
Perhaps a variant Powerwall could control a regular generator, turning the generator on and off, syncing its output to the generator, and controlling its own power output/consumption to ensure that the generator sees a constant load. The generator would need to provide a stable voltage and frequency reference, I'm not sure that many do; and if there is solar, there would need to be some side-channel way of shutting off the solar when production is too great. Alternatively, a special generator with a grid-tie inverter on it could work with a regular Powerwall, although there would still need to be some mechanism to control how much current the generator injects into the AC-grid.
Probably a better solution would be to DC-couple the generator to one of the Powerwalls. That would mean a special Powerwall that has a second AC-input that would be isolated from the AC mini-grid, and which would be used to charge the batteries only. That would make a DC mini-grid within that Powerwall consisting of the batteries, the Powerwall grid-tie inverter, and battery charger. Within various limits on power levels, a system with one of these generator-enabled Powerwalls could recharge other Powerwalls from the generator via the AC mini-grid.
Cheers, Wayne