New Post: Add to List if not in Drop Down

I think SPServices handles Multi-select lookups.  Of course SPServices doesn't add new items to the listbox, but it does identify the element on the form depending on the type of lookup.  Check your reference to SPServices.  The above code assumes you have this file in your layouts dir on web server. "/_layouts/1033/jquery.SPServices-1.7.2.js"  Look in that file for the function 'DropdownCtl()'.  It will give you hints on how to select the listbox using jQuery.

Russ' code add items to the drop downs like so: 

ColumnDropDown.options.add(new Option(this.Title,this.Id)); 

It should be the same syntax for adding to a list box.  Your trouble is getting the correct selector for the list box. (var ColumnDropDown = ?)  Again look at 'DropdownCtl()' function in SPServices.