Two quotes from PreSonus @ Facebook:
"No, the Series III can not be expanded to more than 32 channels. If you need an expandable mixer with more than 32 channel we recommend the StudioLive AI console of rack mixers."
"The StudioLive 32 does not support cascading. However, you can network multiple StudioLive 32 consoles and computers using AVB networking. They won’t operate as a single mixer, as with cascaded devices, but you can control them all with UC Surface."