How to display a control inside an existing control ?

Hello, I've been trying to do something for a while but I don't know what is the best way to go about it. Basically, I have a control (a ListBox), and for configuration purposes, I would like to temporarily show a new control in the middle of it. For example, I have my ListBox, and when the mouse cursor enters it, a ComboBox appears in the middle of this ListBox. When the mouse cursor leaves it, it disappears. The problem is not how to catch these events, but how to handle the control display. Thanks for you help :)