Connect external BLE device

Yes! You probably want to look at https://www.espruino.com/Reference#l_NRF_requestDevice

For instance there's an example there like this:

var gatt;
NRF.requestDevice({ filters: [{ namePrefix: 'Puck.js' }] }).then(function(device) {
  return device.gatt.connect();
}).then(function(g) {
  gatt = g;
  return gatt.getPrimaryService("6e400001-b5a3-f393-e0a9-e50e24dcca9e");
}).then(function(service) {
  return service.getCharacteristic("6e400002-b5a3-f393-e0a9-e50e24dcca9e");
}).then(function(characteristic) {
  return characteristic.writeValue("LED1.set()\n");
}).then(function() {
  gatt.disconnect();
  console.log("Done!");
});

Apart from the NRF bit in NRF.requestDevice, the API itself is based on "Web Bluetooth" so you can always Google that and look for examples.