Re: Adding connectors to obejct exported from AutoCAD.

For the Sketchplane code it won't be used in your case so you can ignore it.

For adding connector in the Family Editor I modified the code to match the updates you sent:

#region Namespaces

using System;
using System.Text;
using System.Linq;
using System.Xml;
using System.Reflection;
using System.ComponentModel;
using System.Collections;
using System.Collections.Generic;
using System.Windows;
using System.Windows.Media.Imaging;
using System.Windows.Forms;
using System.IO;

using Autodesk.Revit.ApplicationServices;
using Autodesk.Revit.Attributes;

using Autodesk.Revit.DB;
using Autodesk.Revit.DB.Events;
using Autodesk.Revit.DB.Architecture;
using Autodesk.Revit.DB.Structure;
using Autodesk.Revit.DB.Mechanical;
using Autodesk.Revit.DB.Electrical;
using Autodesk.Revit.DB.Plumbing;

using Autodesk.Revit.UI;
using Autodesk.Revit.UI.Selection;
using Autodesk.Revit.UI.Events;

//using Autodesk.Revit.Collections;
using Autodesk.Revit.Exceptions;
using Autodesk.Revit.Utility;

using RvtApplication = Autodesk.Revit.ApplicationServices.Application;
using RvtDocument = Autodesk.Revit.DB.Document;

#endregion

namespace RevitAddinCS2
{
    [Transaction(TransactionMode.Manual)]
    [Regeneration(RegenerationOption.Manual)]
    public class ExtCmd : IExternalCommand
    {
        #region Cached Variables

        private static ExternalCommandData _cachedCmdData;

        public static UIApplication CachedUiApp
        {
            get
            {
                return _cachedCmdData.Application;
            }
        }

        public static RvtApplication CachedApp
        {
            get
            {
                return CachedUiApp.Application;
            }
        }

        public static RvtDocument CachedDoc
        {
            get
            {
                return CachedUiApp.ActiveUIDocument.Document;
            }
        }

        #endregion

        #region IExternalCommand Members

        public Result Execute(ExternalCommandData cmdData, ref string msg, ElementSet elemSet)
        {
            _cachedCmdData = cmdData;

            try
            {
                //TODO: add your code below.
                FilteredElementCollector importedElements = new FilteredElementCollector(CachedDoc).OfClass(typeof(ImportInstance));

                Element importedElement = importedElements.FirstElement();
                AddConnectorToFamily(importedElement);

                return Result.Succeeded;
            }
            catch (Exception ex)
            {
                msg = ex.ToString();
                return Result.Failed;
            }
        }

        public void AddConnectorToFamily(Element element)
        {
            // Before acquiring the geometry, make sure the detail level is set to 'Fine'
            Options geoOptions = CachedDoc.Application.Create.NewGeometryOptions();
            geoOptions.DetailLevel = ViewDetailLevel.Fine;
            geoOptions.ComputeReferences = true;
            // Obtain geometry for the given Wall element
            GeometryElement geoElem = element.get_Geometry(geoOptions);

            using (Transaction trans = new Transaction(CachedDoc))
            {
                IEnumerator<GeometryObject> geoObjectItor = geoElem.GetEnumerator();

                trans.Start("InsertConnector");

                while (geoObjectItor.MoveNext())
                {
                    GeometryInstance symInst = geoObjectItor.Current as GeometryInstance;
                    GeometryElement symEle = symInst.SymbolGeometry;

                    foreach (GeometryObject symObj in symEle)
                    {
                        Solid theSolid = symObj as Solid;

                        if (null != theSolid)
                        {
                            foreach (Face face in theSolid.Faces)
                            {
                                PlanarFace pf = face as PlanarFace;

                                if (pf != null)
                                {
                                    XYZ connOrigin = new XYZ(-24.022405238, -59.669659519, 9.583333333);

                                    if (pf.Origin.IsAlmostEqualTo(connOrigin))
                                    {
                                        using (SubTransaction st = new SubTransaction(CachedDoc))
                                        {
                                            st.Start();
                                            ConnectorElement ce = ConnectorElement.CreateDuctConnector(CachedDoc, DuctSystemType.SupplyAir, ConnectorProfileType.Round, pf.Reference, pf.EdgeLoops.get_Item(0).get_Item(0));
                                            st.Commit();
                                        }
                                    }
                                }
                            }
                        }
                    }
                }

                trans.Commit();
            }
        }
        #endregion
    }
}

You may need to replace the FaceNormal property with Normal property to match the Revit 2014 API.

Please notify me if everything is fine.