https://support.docusign.com/en/guides/ndse-user-guide-manager-recipients

The agent (aka Specify Recipients) recipient type should be able to help here.  You can set up the envelope like this:

Known signer at routing order 1

Same known signer as an agent (Specify Recipients) at routing order 2

Unknown signer (with blank name and email) at routing order 3

The agent will be able to enter the details for the unknown signer.

That worked! Thanks very much.

I also need a witness to witness a document being signed. However, I am not quite understanding the instructions in the above link. Can you assist?

This is a pretty old thread. The link I believe directs you to instructions for using the "Specify Recipients" recipient type. It allows one person in your workflow to specify another Recipient. The original case was a Recipient who needed to then provide the Name and Email address of another Recipient. The design laid out in the original thread was:

Recipient 1 - Needs to Sign, known signer at routing order 1

Recipient 2 - Same as recipient 1 but set to "Specify Recipients" at routing order 2

Recipient 3 - Unknown signer (has Role but with blank name and email) at routing order 3. Name and email address to be added by Recipient 2

Is this you situation?

I am also facing the same issue, it's mandatory to fill out the witness's email id which is not known to me before I send out the envelope