You absolutely can! When connecting your first Plume Pod to a modem or router, the pod will be set to "Router" mode. You can set the rest of your Pods up in this mode without issue. You will want to change this to "Auto" mode using your iOS or Android app once all of your pods have been claimed and your Plume network is up and running.
In "Auto" mode, Plume selects "Router" or "Bridge" mode depending on what type of device (mode/ router/ combo) you connected your Pod to and whether that device is managing your network already. Please note that switching the Plume mode from "Router" to "Auto" mode will cause all of your Pods to reboot.
If you do connect Plume to your existing router, do not forget to turn off the WiFi signal on your original router after you've set up Plume. It is important that you use a different name and password for your Plume WiFi network to make sure that your devices don't get confused and try to switch back and forth between your new Plume network and your older router's network. Please refer to: How do I turn my existing router's Wi-Fi off? for further instructions!