var select_box = document.getElementById("mySelectBox");
for(...){
	var field = document.createElement("option");
	field.setAttribute("value","somevalue");
	field.appendChild(document.createTextNode("lalalal"));
	select_box.appendChild(field);
}



// Jetzt mit JQuery
for(...){
	$("#mySelectBox").append('<option value="somevalue">lalalal</option>');
}