How do I get a new incident to assign a contact based on the email from Ask-a-Question?

Summary

Content (required):

We do not require users to be logged in to ask us a question in the customer portal. However, they do have to give us an email address. If the email address exists in the Contacts object, I would like the incident to have the complete contact record associated with it based on the email address when the agent opens the incident. This will spare the agent having to look it up.

Is this possible? I didn't see a way to take the email value that is showing in the Contact tab in the incident and in the Contact ID field in the incident and use it to search for the customer contact, and select it. I checked business rules, workspace rules, and workflow and didn't see a way to get the system to automatically assign the customer contact (if it exists) based on the email address.