Several issues here.
First, the AVR won’t put out surround information via PreAmp RCA jacks
Second, any analog line in on the Sonos will be slightly delayed by 75ms.
As @Stanley_4 suggests, the Sonos Port is only stereo capable. If this were to work (extremely poorly, and not suggested), the input to the Port would need to be the surround channels, and not a stereo output from the PreAmp outputs. Most AVRs don’t provide a line level surround output, only an amplified version, which would connect to normal speakers.
Sonos doesn’t make any solution for adding only surround speakers. You either need to go all in with a Sonos soundbar, or not at all. Perhaps unfortunate, I don’t know what sales would be, but the current state of affairs.