Actually, I believe you can.
There isn't a 50-amp "fusion" box yet. There is a setting in the menu that limits the AC500 power to 3000w in order to work with the existing AC300 30-amp fusion box.
There is a video on Facebook of a guy doing a 80-min video of the AC500 review where he shows this. It doesn't say explicitly that it can be done, nor does he tests it, but he says what the setting is for and why it's there.