You can do this in the skin using jQuery.
You can hook on the submit button and add an additional check on the field that you want to make required. You can also use jQuery to add the red required "*".
Example: add some javascript like this at the end of the head section of template.php:
$(document).ready(function(){
$("body.checkout_shippingoptions #fcReferenceLabel").append('<span class="asterisk">*</span>');
$("body.checkout_shippingoptions #saveButton").click(function() {
if ($("#fcReferenceField").val()=="") {
alert("No value");
return false;
}
else
return ajaxSetSaveButtonGlobal();
});
});
ps. note I only checked this on a PSM Web 7.0 system (but it should work the same on a PSW2.