Let’s discuss the Fronius Primo and Symo for example they are not hybrid inverters. They are grid-connected inverters the same as Sungrow SG5k-D etc.
The only way you can have a battery added to these inverters at a later date is with an A/C coupled battery such as Tesla which has its own inverters inbuilt into the battery. They are considered another grid connection rated at what the battery inverter capacity is.
So, with a tesla, it is considered another 5kW connection at the premise.
The exception to this is the Solarwatt batteries which can be connected to a grid connect inverter but have no backup capability.
These options don’t allow for additional panels.
Hybrid inverters however allow for batteries to be directly connected to the inverter. This is D/C coupled and when the battery is added, more panels can also be added with STCs claimable.
It provides a single conversion point for all incoming and outgoing power flows, has backup, and is considered more efficient as the power conversion all occur at a single point.
Example Enphase micro Inverters, Sungrow hybrid single-phase and three-phase inverter, Fronius hybrid single-phase and three-phase inverters, etc.
A/C coupled systems can go on any existing solar system, but they are considered a secondary system with a secondary conversion point. This is generally how others suggest that a battery can be connected at a later date, but the difference is, it is a completely separate system with a new connection to the grid.