To move the stand alone sensors to a zone, first you have to go to Settings->Sensors->Edit(upper right corner) and then delete the sensor. Next you will have to go to the home page. There is a drop down menu and please select your Pixie, then tap the "+" button on the upper right corner,  go to Add a Product-> Plant sensor and then complete the following procedures to add the sensor. You will need to press the button 6 times on the back of the sensor to change its mode to Setup mode (light blinking slowly).