Maybe I'm confused about what you need to do, but you don't need BEC on the Rx to use the ESC's BEC.

The logic circuits in both ESC and Rx need about 5v (4.8 - 6.0v battery pack will do).
Some Rx's have "BEC"... merely means it has an internal voltage regulator that allows you to plug in a lead directly from your main pack (7.2v? 8.4v? etc); the regulator makes 5v and sends that to the Rx brain, and on to the servos as well.

But most ESC's do the same thing... as as long as they're regulating 5v out of the main pack, and as long as the ESC already has a lead going to the Rx with signal and ground, why not just send some of that 5 volts up the red wire back into the Rx?

So... for BEC or non-BEC Rx's, just connect that BEC-enabled ESC to the Rx, and away you go.

Now, should you prefer to use a separate pack for your Rx (either a 4.8-6v Rx pack, or any higher V pack into a BEC-Rx), it's common to disconnect the 5v lead out of the ESC. You can just snip the red wire (crude), or use a sharp little tool to unhook the 5v lead out of the plastic shell, and tape it back... so that you can use BEC later should you change your mind. That's all there is to it.

Pat M