Yes! For the best network performance, additional eeros can be connected via a wired ethernet connection.
Each additional eero will then extend the Wi-Fi network and use the wired connection for its connection to the internet.
It is worth bearing in mind that connecting eero devices together via a wired connection will require using one of the two Ethernet ports on the back of each device. This will of course reduce the number of devices you can connected to the eero via a wired connection.