I think it's going to be a coding effort on your end to get this going. It's not something that's used out of the box (e.g. route ESC voltage as a battery voltage).
My only experience has been when we created our power module driver, here:
https://github.com/PX4/PX4-Autopilot/tree/master/src/drivers/power_monitor/voxlpm
On the top of my head, you could modify the driver above, instead of querying I2C to our power module, you could just subscribe to your ESC messages in there, and covert that to a power monitor message perhaps!
More info:
https://docs.px4.io/master/en/config/battery.html