Print the extension forms to PDF.  Start the DocuSign as usual. In the screen that lists document types and names, use the Select Document Type drop-down to pick Other Document for ESignature. Browse to the PDF extension forms and attach them (one file at a time, so if you have them in two PDFs, do it twice).  Delete the automatically created documents for the actual return filing (one of them will be the 8879, the other will be the 579).  Say Next.

Click through the remaining screens as normal. You'll eventually be taken to a DocuSign web browser screen where you'll be able to stick signature and date tags on the extension forms manually.