Virtual LEDs allow you to "attach" LEDs from multiple ESPs to a main, controlling ESP. These LEDs can be added to an ESP just like physical pins.
On the controlling ESP, go to Config, LED Preferences.
Select DDP RGB (Network) as the LED type and enter the Length (number of LEDs on that remote ESP), then enter the ESPs IP address.
Multiple remote ESP's can be setup this way.
Note: The controlling ESP must be running at least 0.13 firmware while the remotes can be older. As usual, best perfomance is obtained by using an ESP32 for the controlling device. You can use an ESP8266, but only with a small number of LEDs (<300).
Also, support was recently added for ArtNet virtual LEDs.