It is possible, using the IUIElementCreationFilter interface. Note, however, that you might have a problem with the amount of space required to show the date range and the week number. One possible solution for that problem is to use the VisibleWeeks property to reduce the number of weeks shown by the control, thus increasing the vertical space available to each one. The following code sample demonstrates this approach:

using nsMVS = Infragistics.Win.UltraWinSchedule.MonthViewSingle;

this.monthViewSingle.VisibleWeeks = 4;
this.monthViewSingle.WeekHeaderDisplayStyle = WeekHeaderDisplayStyle.DateRange;
this.monthViewSingle.CreationFilter = new WeekNumberCreationFilter();

public class WeekNumberCreationFilter : IUIElementCreationFilter
{

    #region IUIElementCreationFilter Members

    void IUIElementCreationFilter.AfterCreateChildElements(UIElement parent)
    {
        if ( parent is nsMVS.WeekNumberUIElement )
        {
            Week week = parent.GetContext( typeof(Week) ) as Week;
            TextUIElement textElement = parent.GetDescendant( typeof(TextUIElement) ) as TextUIElement;
            if ( textElement != null )
            {
                string text = string.Format( "{0}-{1} ({2})", week.FirstDate.ToString("MM/dd"), week.LastDate.ToString("MM/dd"), week.WeekNumber );
                textElement.Text = text;
                textElement.Rect = parent.RectInsideBorders;
            }
        }
    }

    bool IUIElementCreationFilter.BeforeCreateChildElements(UIElement parent)
    {
        return  false;
    }

    #endregion IUIElementCreationFilter Members
}