The Amp doesn’t ‘serve’ a center channel. When using the HDMI input (and a 5.1 input), it creates a faux center channel, interpreted by the front right and front left speakers. There have been no reports from people who find this objectionable, but that’s the only ‘center’ channel it supports.
I don’t think the Sonos Amp is going to support any wiring to a center speaker.
I looked into this when it first came out but haven’t read the manual / specifications since but I believe it hasn’t changed.