First of all, is your existing array generating enough spare solar power to fill a battery for you?

Secondly, you may need a new AC coupled or hybrid inverter to match your battery (DC storage) with your existing solar.

Thirdly, for liability reasons, your battery installer may be unwilling to hook up their equipment to an older system they have had no responsibility for. They would probably prefer to install a new solar PV system parallel to your existing system or to replace your existing panels altogether. If your solar array is relatively new, go back to your installer and get a quote from them first before considering other installers. To be eligible for the battery rebate you do need to have some solar panels installed at your property.

More information: The Truth About 'Battery Ready' Solar Systems

Can You Add A Battery To Your Existing Solar? Here's How To Tell