If you can read sensor data on your raspberry pi, then you can send the data to cayenne.
you can also connect SonOff to cayenne, have a look at this post Solderless SonOff Basic working in Cayenne Over The Air
this entirely depends whether you can read the voltage-current on the raspberry pi. once that is done you can use the cayenne python library to send this data to cayenne,
yes, use the python code from the tutorial to read sensor data and send it to cayenne using GitHub - myDevicesIoT/Cayenne-MQTT-Python: Python Library for Cayenne MQTT API