Convert Reporter to Customer after Customer Creates Account

[Kristin Lyons]

We have a JSM portal that is open to anyone and we have a lot of requests that come in on a daily basis.  Currently we create the issue with the agent as the reporter and then add the customer's email address to a custom field.  For customers who sign up as a customer after the issue is created, is there a way to automate a process where it would detect if that email address is being used for a customer and allow the customer to access the issue in the portal?