Adding an EBEC to a system wich has a ESC with bec.

[Shotglass]

yes that is the way to do it
signals are usually always ground referenced so youll want to cut the positive wire

btw in case you may want to use the esc for something else later using a servo extension wire and cutting that instead of the wire of the esc itself will save you some headaches down the road

[Flash1940]

Or.....if you're meticulous....you could remove the red wire from the connector by pushing down on the locking tab with a sharp pointed object while pulling gently on the red wire. Fold it back & tape it up for another day....
Flash

[quarry44]

Just like this: