They're wiring through a Surepower 2 amp, 3 bank battery
isolator.
Missing a couple of zeros?

That should be '2 alternator, 3 bank batter isolator' not amp.

Can I just add this new bank in parallel to the existing ships system
battery?
Yes
Do I need to add another isolator?
No so the following is invalid

No because an isolator won't work, or some other reason?