If the control is a .NET control you may be able to create a UI connector to accomplish this provided you are on a version of Robotics greater than 8.0.1053. Otherwise, I am not aware of a manner in which you can accomplish this task.
Information on creating a UI connector can be found here:
I couldn't find much on the "handled" argument but maybe this will help... the green block doesn't show the parameters by default but you can expand the event to show everything. Is this what you were looking for?