Sadly, I don't think there's any way to programmatically insert a rich symbol while maintaining its "richness".  I tried and failed to do that in my QuickFire extension: http://johndunning.com/fireworks/about/QuickFire

The best I could do was to insert the symbol in a way that if you had already dragged that symbol from the common library into the document, the new instance would still show something in the symbol properties panel.  Here's the code that QuickFire uses to insert symbols:

function insertSymbol(
     inSymbolName,
     inPath)
{
     var dom = fw.getDocumentDOM();
     var middleX = Math.round(dom.width / 2);
     var middleY = Math.round(dom.height / 2);

     try {
               // first assume the symbol is already in the library and try to 
               // insert an instance.  there doesn't seem to be an API to list
               // the symbols in the document, so we just have to try it.
          dom.insertSymbolAt(inSymbolName, {
               x: middleX,
               y: middleY
          });
     } catch (exception) {
               // replace the user or app JS directory path tokens with the 
               // actual path, so that importSymbol will find it below
          inPath = inPath.replace(/^:U:/, k.UserSymbolsPath).replace(/^:A:/, k.AppSymbolsPath);
          
               // the symbol isn't in the document yet, so import it.  we use
               // importFile instead of importSymbol because the former seems
               // to preserve some magic that allows you to turn it back into a
               // rich symbol if you later drag the same symbol from the Common
               // Library into the doc and replace existing instances. 
          dom.importFile(inPath, {
               left: middleX,
               right: middleX,
               top: middleY,
               bottom: middleY
          }, true);

// this is a failed attempt to import a rich symbol. 
/*
               // the jsf filename doesn't include the .graphic bit
          var jsfPath = inPath.replace(/\.(graphic|animation|button)\.png$/, ".jsf");
          
          if (Files.exists(jsfPath)) {
                    // to change the opCode, so that the rich symbol's jsf file
                    // will give it the default values, we need to create a proxy
                    // Widget object.  but first save off the actual Widget.
               var _Widget = Widget;
               
               Widget = {
                         // we want this to be opCode 1, which will set the symbol's
                         // default values 
                    opCode: 1,
                         // the symbol we just imported is Widget.elem
                    elem: fw.selection[0],
                    propString: _Widget.propString,
                         // pass these method calls on to the real Widget
                    GetObjectByName: function()
                    {
                         return _Widget.GetObjectByName.apply(_Widget, Array.prototype.slice.call(arguments, 0));
                    },
                    isWidgetSelected: function()
                    {
                         return _Widget.isWidgetSelected.apply(_Widget, Array.prototype.slice.call(arguments, 0));
                    },
                    UpdateWidget: function()
                    {
                         return _Widget.isWidgetSelected.apply(_Widget, Array.prototype.slice.call(arguments, 0));
                    }
               };
               
               fw.runScript(jsfPath);
               
                    // reset the global to the real Widget object 
               Widget = _Widget;
          }
*/
     }
}

Hope that helps.