このコードがこれを実現するのに役立つことがわかりました。
Document oldDoc = itemToReplyTo.GetInspector.WordEditor;
Document newDoc = newMailItem.GetInspector.WordEditor;
if (oldDoc != null)
{
Microsoft.Office.Interop.Word.Selection oldSelect = oldDoc.Windows[1].Selection;
Microsoft.Office.Interop.Word.Selection newSelect = newDoc.Windows[1].Selection;
oldSelect.Find.Execute("From:");
oldSelect.Collapse(WdCollapseDirection.wdCollapseStart);
oldSelect.MoveEnd(WdUnits.wdStory, 1);
oldSelect.Copy();
newSelect.Move(WdUnits.wdStory, 1);
newSelect.InlineShapes.AddHorizontalLineStandard();
newSelect.Paste();
newSelect.Move(WdUnits.wdStory, -1);
newSelect.InsertAfter("Reply Text here");
newSelect.Find.ClearFormatting();
newSelect.Find.Execute(mailItem.SenderEmailAddress);
}