Thank you, it is not what I needed but I found a solution (I need to know on what field user doubleclicked, not what value have the field).
Here is how I use it now:
$.extend($.fn.numberbox.defaults.inputEvents, {
dblclick: function(e){
var t = $(e.data.target);
var id = t.attr('id');
alert(id); <- Here I see my field "id", like "first_name", "last_name" etc.
}
});
Thank you!