private ObservableCollection<double> thick;
        public ObservableCollection<double> Thick
        {
            get
            {
                if (thick == null)
                {
                    thick = new ObservableCollection<double>();
                    for (double i = 0; i < 10; i++)
                        thick.Add(i);
                }
                return thick;
            }
            set { thick = value; }
        }

<ComboBox Width="70" Name="combo" ToolTip="Выберите толщину границы"
                          ItemsSource="{Binding Path=Thick, Converter={StaticResource doubleConverter}}"
                  SelectedItem="{Binding Path=Poly.StrokeThick, Converter={StaticResource pathConverter}}" 
                          SelectionChanged="combo_SelectionChanged"/>

private double strokeThick = 1;
        public double StrokeThick
        {
            get { return strokeThick; }
            set {
                strokeThick = value;
            OnPropertyChanged("StrokeThick");
            }
        }

[ValueConversion(typeof(double), typeof(System.Windows.Shapes.Path))]
    public class ThicknessToDoubleConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            //не работает
            var thickness = (double)value;
            System.Windows.Shapes.Path myPath = new System.Windows.Shapes.Path();
            myPath.Stroke = Brushes.Black;
            myPath.StrokeThickness = thickness;
            GeometryGroup myGeometryGroup = new GeometryGroup();
            // Create the line geometry to add to the Path

            myGeometryGroup.Children.Add(new LineGeometry(new Point(0, 10), new Point(60, 10)));
            myPath.Data = myGeometryGroup;
            return myPath;
        }

        /// <summary>
        /// конвертирует линии в значение их ширины
        /// </summary>
        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            var myPath = (System.Windows.Shapes.Path)value;
            return myPath.StrokeThickness;
        }
    }