Migrating to the STM32F405ZGT6

Ahh - so you want to implement the terminal, but also interpret AT commands?

You can't access the port the terminal is on, but you can move the terminal to a 'loopback' port where you can send data to it - so for example:

LoopbackA.setConsole();
USB.on('data', function(s) {
  if (isAnATCommand) doStuff(s);
  else LoopbackB.write(s);
});
LoopbackB.on('data', function(s) {
  USB.write(s);
});