Can't "add on" ... or can't find someone who will merge two systems.
According to Tesla at least
"Unfortunately Tesla solar panels cannot be added to any third party solar system, you can however add more Tesla solar panels to an already installed Tesla solar system."
I would find someone else or negotiate with Tesla. There's no reason you can't parallel two PV systems... it's no different than if you have PV and so does your neighbor... just less distance. Lots of options out there.