Adding More Properties to Standard Inline Sublists

We want to extend the properties of the 'assignee' sublist server side.  We see that we can add the values to the sublist and retrieve them.  But as soon as we submit the values to the database and then retrieve them again, we lose the new extra values.  Ideally, we could extend the properties of this object because it fundamentally just seems to be an array.

See following code;

 / try saving data into the new fields  function customizeProjectAssignee_TaskafterSubmit(type)  {             var newId = nlapiGetRecordId();     var newType = nlapiGetRecordType();          //load the task record     var loadedTask = nlapiLoadRecord('projecttask', newId);          nlapiLogExecution('DEBUG','<After Submit Script> type:'+type+', RecordType: '+newType+', Id:'+newId);     //nlapiLogExecution('DEBUG','loadedTask startdate:'+type+', RecordType: '+newType+', Id:'+newId);                    // load the task record   var assigneeCount = nlapiGetLineItemCount('assignee');            // output assignment row count      nlapiLogExecution('DEBUG', 'customizeProjectAssignee_TaskafterSubmit. assigneeCount', assigneeCount);              // prove that we are looping the data; show the service items   for ( var counter = 1; counter <= assigneeCount; counter++ )   {    var serviceitem =  loadedTask.getLineItemValue('assignee', 'serviceitem', counter);          nlapiLogExecution('DEBUG', 'serviceitem', serviceitem);            // this should work -- but not saving