Yes, it can. I found myself in the same situation but felt that leaving the old Google Wi-Fi as the main router was defeating one of the greatest benefits the new Nest Wi-Fi solution offers: much better bandwidth if the Nest Wi-Fi Router is used as the main router.
To do this, I Reset the old Google Wi-Fi using an option in the old Google Wi-Fi app - which you have to do if you plan on adding the old units into this new environment. I then added the Nest Wi-Fi Router in as my main router using Google Home and named the network exactly the same as my old network - to include the same password. By calling this new network the same as the old and by using the same password, all your devices will immediately connect to the new network - albeit, with new IP addresses. I then set up the Nest Wi-FI Point using Google Home, followed by adding two of the old Google Wi-Fi units. In doing this, I now have the Nest Router as my high-performance router (much faster than the old one), and a total of 4 devices, i.e. Nest Router, Nest Point, and two of the three Google Wi-Fi units. I didn't need the third so I'm keeping it aside in case one breaks.
Still, you can add the new Nest devices into your old network configuration if you want but will miss out on the benefits the new Nest offers.