How to restrict Signatory selection from a list

Hi,

Need Help with the following Usecase. 

We want to restrict  the signatory who can  be selected from Signing. The people who can be selected as a signatory can be maintained in a list. How can this be done ?

Any public list company has a list of people who are authorise to sign documents on behalf of the company. So when an employee upload a document for Signing, We want the person to select only from the list of people (we can maintain the list of email ids for List of Authorised people who can sign). Can this be done via workflow ? Workflow does not allow selection from a list as far as i know.

Thanks

Gautam