I'm facing a similar / the same problem and decided to use a small boost converter to convert 5V to 9V and feed that into the Arduino's Vin (Vraw) / barrel.
You can not feed 5V into Vin (Vraw) / barrel as the voltage drop over the regulator will lower the Vcc for the boards electronics to a level that s putside its specs.
PS
I'm using a Mega, but the above applies to all boards.