Risolto
codice:
string altChunkId = "AltChunkId1";
            MainDocumentPart mainPart = document.MainDocumentPart;
            AlternativeFormatImportPart chunk = mainPart.AddAlternativeFormatImportPart(
                AlternativeFormatImportPartType.WordprocessingML, altChunkId);
            using (FileStream fileStream = File.Open("embed.docx", FileMode.Open))
            {
                chunk.FeedData(fileStream);
            }
            AltChunk altChunk = new AltChunk();
            altChunk.Id = altChunkId;
            body1.InsertAfter(altChunk, body1.Elements<DocumentFormat.OpenXml.Wordprocessing.Paragraph>().Last());