Now when you say "External BEC" you mean a battery pack correct?
Fun with terminology.
An external BEC is more properly called a Voltage Regulator. You pair a battery with the Voltage Regulator and use it to power your electronics. This is separate from the battery that runs your motor.
When the Voltage Regulator is included in the ESC, it is more properly called a BEC or Battery Elimination Circuit, because then you run everything off one battery and you have truly eliminated a battery.
On the other hand, another way to set this up is to connect the Voltage Regulator to the main battery along with the ESC power wires. You'll have to splice them together. In this configuration, you've still eliminated the receiver battery, so there's only one main battery to deal with, BUT you have also relieved the ESC of the duty to provide power to the electronics. So you have the reliability of an external Voltage Regulator, but the simplicity of a single battery.
You just need to make sure the Voltage Regulator you use can take the voltage of the main battery pack. One which expects to get 8.2v from a 2S Receiver Battery Pack might blow up if you connected it to a 12S battery at 50.4 volts fully charged.