You can definitely ”upgrade” your system, by which I mean you can add a new system of 24V Poly panels. However, your new system may not be compatible for connecting to your existing system. This is because your old system has a lower rating. So, connecting to your old system basically depends on the inverter and charge controller (assuming it is not integrated). I am assuming that your charge controller is 12V, so, for 100W panel system, it needs to have 8.34Amp rating. For a 24V system, you need to have 4.16Amp rating. You might have to replace your inverter system or setup a parallel system that integrates with your existing system.