Allow one signer to specify another signer of fields when completing a form

In some instances, one of the signers needs to determine who another signer should be and this will not be known when the form is initially submitted. For instance, you may have a form that an admin or coordinator submits to a guardian to sign that also requires the signature of an unknown 3rd party like a physician. If there was another signer type and mechanism that would allow the guardian to dynamically assign the email address of that signer when they signed, that would allow the form to work in those instances.