Interesting... how to concactinate a nothing. Funny whe you think about it philosophically.
But back to your question, I would check with the guys at TROI (www.TROI.com) because they make a serial port plugin that I have used with phone systems and I bet they would know how to do this.
Perhaps an escape sequence for Null might work? (e.g.: \0, \z, or \x00 )