Specialized WhatsApp tools such as Whatsapp Xtract exist and allow users to export the conversations from the original WhatsApp backup files (.crypt) and save them either as HTML or even directly as MS Word documents (.doc, .docx), hence effectively allow for crypt to doc conversion, although it is not a file conversion in proper sense.
Once you have either of these files, you can easily get a .doc file by resaving the document in MS Word.