Unfortunately most companies are not touching another installers system to expand on. Its simply too much liability and headache for a small profit to take ownership of a complicated Solar plus PW job that you didn't initially do, and do not know the health of. You may have better luck as an owner/builder and do the expansion yourself with some smart help.
Especially given that many older systems will not pass NEC 2017 muster, this is a reasonable business decision. PV companies are looking to make money, and limit risk. If you expand it, you are changing it, and so AHJ are within rights to make you update it to the current version of rapid shutdown (starting Jan 1 in California), at significant cost for some systems.
However its not excusing Tesla from honoring their commitments. I'd recommend starting up with the "Squeaky wheel" treatment, and make it more of a headache for them to ignore you than to serve you. Certainly systems that we install we will upgrade or expand, as long as it is a significant expansion.
If you want to do it yourself, just throwing a few more KW onto an existing inverter is possible but adding 2-3k onto a loaded up 7k will create a bunch of extra heat. This will effect the life of the inverter as well as the warranty. There is also maximums to consider with string lengths, max DC power and such. I'd recommend if you understand the design well enough to answer these questions, you hire an independent PV designer to build new plans and bring a revised design to a small contractor and see if they are hungry to do this work under your owner/builder permit (if you are comfortable with that).
Most PV installers are slammed with the Powerwall surge from PGE outages, and the end of year push to get the 30% tax credit. Many are booked out 4 months or more.
