Attaching Written Signature PDF to Adobe Signed PDF

Is there a way to upload a written signature PDF to an existing signed PDF?

I have an issue whereby all recipients signed except for one person. He manually signed and sent me that PDF via email. I need to add that PDF to the PDF in Adobe that has the others that signed via Adobe Sign. The document is still in my 'Out for Signature' box since he didn't sign via Adobe Sign. How can I do this?

Thank you for your assistance!