Add the existing panels and inverter to the system design, just as you would for quoting new ones.
Then on the Solar and Battery designer page, click the 'pre-existing' box next to each inverter and MPPT. That way their energy production is factored into the site and battery charge, but they are not added to the quote.
The consumption profile on the Energy page needs to be “before solar”, i.e., the customer’s actual consumption - not the nett energy imported from the grid (ie reduced consumption due to existing solar).
The solar export earnings from the existing system do not contribute towards the payback/ROI of the battery
The payback periods for battery only upgrades on existing solar are usually quite long.