The Roku mobile app can only control one device at at a time so really not an "Add" feature, but if all your Roku devices on the same network, you should be able to switch between them easily. On the bottom right, should be a "Devices" tab/selection. All the available Roku devices you can connect to should be located there.
If you are currently in "Remote" mode, press the X button in upper left corner and you should be presented with "Roku Channel", "Remote" or "Devices" options. On Android, the list of available devices automatically appears when not in "Remote" mode. You can also pull down on screen to refresh connections.
Roku Community Streaming Expert Just another Roku user... I am not a Roku employee. Insignia RokuTV, Ultra 4660, Premiere+ 3921, Express 4k+ 3941, Streambar 9102