Arduino USB Serial Extension - asynchronous read

In the USB extension, the read and write are asynchronous. This is what I would like to do. When a button is clicked, the content of a text box is sent to the arduino. Arduino will do some processing and return the result after a few milliseconds - the time elapsed can vary. Right now the extension does not have a way to read the data when it arrives. Is it possible for you to modify the extension so that an interrupt routine senses the arrival of data at the USB terminal and show the result such as in the web we have a .GotText method?