Found the Gravitech USBHOST add-on for Arduino Nano: http://www.gravitech.us/usadforarna.html

Figured out how to make it work and now an Arduino Nano can now talk to an Arduino Uno thru USB

Demo below shows a text being entered in the enigma simulator, it uses Serial.print to send it through USB, the Arduino nano receives it thru SPI using the USB Host shield and flashes the morse code of the encrypted letter. 

https://www.instagram.com/p/BW2ita-hWNu/

Source:

https://github.com/arduinoenigma/UsbHostMorseLED