Yes indeed! You can set a DHCP IP reservation for any device connected to your Plume network as long as your Plume Pods are operating in "Router" mode. You can find this setting in both the iOS and Android apps within the "Advanced Settings" page.
If your Plume Pods are operating in "Bridge" mode you will not be able to set up DHCP IP reservations through Plume as your original router or modem is managing all of your network settings. In this mode, Plume is simply ensuring the Wi-Fi coverage in your home is the best it can be.