Best answer by Airgetlam
A Sonos sub could easily be added to the PLAYBAR, again with no need for a receiver.
The issue comes when you want to use existing speakers for the surrounds. This requires use of a CONNECT: AMP, which has to be connected via an Ethernet cable to the PLAYBAR ( or your router, and the PLAYBAR needs to be connected via cable as well)
Here’s an FAQ on this site that explains how to set up a CONNECT:AMP as a surround driver.
That set up would give you the 5.1 you’re looking for. I’d honestly be tempted by ditching the existing speaker and getting a pair of Sonos Ones to use as surrounds instead, but that’s just an opinion.