Wouldn't this equal a textbox that can be filled with what you want? If you mean a listbox presenting a set of constraints and one of them being an option for entering free text… not yet (unfortunately).
In fact in a field 90% of the values entered wil be in a very short list. very rarely, the field needs a value which is not in this short list. I want to suggest to the users this short list and leave them the possibility to add other values.