This isn't exactly what you're asking, but here's what we did:

• collect signatures through Forms
• create a "final" form with all signatures that could be saved to the Repository as a PDF "Signature Page"
• use Workflow to find the main PDF and the associated signature page, and then use a PDF Merge utility within Workflow to merge them into one document (we purchased PDF Merge here: https://qfiche.com/products)