Could you tell me a little more about what you are trying to do? Do you mean you want to do that on the Arduino?

You would make a custom node, see this section of the docs.

Basically:

• Go to the Project tab (the tool tab with the Embrio logo).
  
• Right click and make a new node definition.
  
• Drag the definition onto a node screen, then open it up for editing by pressing the open arrow in the top right.
  
• Click "touches pin" so that the node only runs on the Arduino.
  
• Then the node code would be very similar to a simple Arduino program. In the setup handler you would do a serial.begin(), then in the Every Update handler you would do a serial.Read(), and if it gets data, send it out with a Serial.Print().
  

You might run into an issue though. While you're working in Embrio with a connected Arduino, they communicate over the serial port, so what you write might interfere. However in a final compiled program it shouldn't be a problem.

Is this part of a larger project? What else will you have going on?