I have a Fronius inverter. Can I add an Enphase system to another available 2-pole breaker without causing problems?
If grid power isn't available, my Fronius shuts down. Does the Enphase shut down as well? If so, I don't see any conflicts if I add a few panels with Enphase micros and connect them to a separate available 2-pole circuit breaker on my house panel box in the basement.
Yes, in theory you can expand an existing PV system. You still have to make sure your AC panel bus will meet the NEC 690.64(B) requirements with the additional PV back feed breaker.
The Enphase inverters also shut down if the grid power goes down.