Word.Range range = application.Selection.Range;
Word.Document docAdd = application.Documents.Open(path);
docAdd.Content.Copy();
docAdd.Close();               
range.PasteAndFormat(Word.WdRecoveryType.wdFormatOriginalFormatting);

<как-то переходим в конец вставленного текста ???>

<далее пишем добавочный текст..>

Word.Range range = application.Selection.Range;
Word.Document docAdd = application.Documents.Open(path);
int p = docAdd.Paragraphs.Count;
docAdd.Content.Copy();
docAdd.Close();               
range.PasteAndFormat(Word.WdRecoveryType.wdFormatOriginalFormatting);
Word.Range r = range.Next(Word.WdUnits.wdParagraph, p);
r.Select();