How to convert a WordprocessingDocument to a byte[]?

Question

I have a WordprocessingDocument that I clone into a new object using the following function:

private static WordprocessingDocument CloneWordprocessingDocument(string docPath)
{
    using (var fileStream = new FileStream(docPath, FileMode.Open, FileAccess.Read,
               FileShare.ReadWrite))
    {
        using (var originalDoc = WordprocessingDocument.Open(fileStream, false))
        {
            var clonedDoc = originalDoc.Clone() as WordprocessingDocument;
            originalDoc.Close();
            return clonedDoc;
        }
    }
}

Afterwards, I process the cloned object using various methods. This procedure can be repeated for a large number of objects, if necessary, which are subsequently kept in a list.

These objects are then to be merged into a single .docx. Currently I write them on disk for this and pass the paths to DocBuilder.

public static string MergeDocs(List<string> docPaths, string tmpPath)
{
    List<Source> sources = docPaths.Select(docPath => new Source(new WmlDocument(docPath), false)).ToList();
    var finalDoc = DocumentBuilder.BuildDocument(sources);

    var finalPath = tmpPath + Guid.NewGuid() + ".docx";
    finalDoc.SaveAs(finalPath);
    return finalPath;
}

To save myself the overhead of storing them, I would like to convert them to a byte [] and pass them to DocBuilder that way.

I know that I can initialize a WmlDocument with a byte[]. But how can I convert a WordprocessingDocument to a byte[] without saving it?