[atholon]

You could do something like this in javascript:

function DropDown(targetName)
{
	var dropDownInst = this;
	this.target = document.getElementById(targetName);	
	
	if(targetname == null || this.target == null)
		return;
	// better to add some sort of exception handling here
	
	// Create prefix array
	this.prefixes = ["prefix1", "prefix2", "prefix3"];	
	
	// Create DOM select object
	var prefixSelect = document.createElement("select");	
	this.addOptions(prefixSelect, prefixes);
	
	// This is where we handle adding the item
	prefixSelect.onchange = function(e){ dropDownInst.addItem(this); };
}
DropDown.prototype.addOptions = function(iSelect, iArray)
{
	for(var i=0; i<iArray.length; i++)
	{
		var option = document.createElement("option");
		option.value = iArray[i];
		option.text = iArray[i];
		iSelect.options.add(option);
	}	
}
DropDown.prototype.addItem(iSelect)
{
	var selectedValue = iSelect.options[iSelect.selectedIndex];
	
	switch(selectedValue)
	{
		case "desiredPrefix1":
			// Add code here to create a new select and append it to the target
			break;
		case "desiredPrefix2":
			// Add code here to create a new select and append it to the target
			break;
		case "desiredPrefix3":
			// Add code here to create a new select and append it to the target
			break;
		default:
			alert("An invalid selection occurred!");
	}
}